[SOLVED] bPopup opens centre-aligned in one page and left-aligned in other

I'm using the bPopup plugin to open an iframe on two separate pages.

I've written a small script which creates the iframe and "pops it up". This same script (exact URL etc) is referenced on both of my pages:

jQuery('#student_iframe_container').css({
      'display': 'none'
});

jQuery('body').on("click", ".student_iframe_popup", function() {
      jQuery('#student_iframe_container').remove();

      var popUpHTML = '<div id="student_iframe_container"><div class="b-close"><img src="/user/74/187888.png" title="Close" /></div><iframe></iframe></div>';
      jQuery('body').append(popUpHTML);

      jQuery('.b-close').css({
            'background-color': 'none',
            'padding': '8px',
            'font-size': '14px',
            'font-weight': 'bold',
            'width': '52px',
            'text-align': 'center',
            'cursor': 'pointer',
            'overflow' : 'auto'
      });

      jQuery('iframe').css({
            'margin': '0px auto',
            'background-color': 'none',
            'border': 'none'
      });

    var student_id = 'student_' + jQuery(this).attr("id");

    jQuery('iframe').attr("width", 800);
    jQuery('iframe').attr("height", 670);
    jQuery('iframe').attr("id", student_id);
    jQuery('iframe').attr("src", '/index.phtml?d=825090');

    jQuery('#student_iframe_container').bPopup({
            position: ['auto', 2]
      });
});

The idea of this script is that I can whack <a href="javascript:void(0);" class="student_iframe_popup" id="123456">student</a> into my pages and the script will do the rest.

On both of my pages the pop-up iframe works correctly. The issues lies with alignment: on one page the iframe correctly appears horizontally centre-aligned but on the other it appears left-aligned.

Correct alignment on Assign Flags page

Incorrect alignment on My Flags page

If I look at the HTML in FireBug it's fairly messy:

Assign Flags:

<div class="b-modal __b-popup1__" style="background-color: rgb(0, 0, 0); position: fixed; top: 0px; right: 0px; bottom: 0px; left: 0px; opacity: 0.7; z-index: 9998; cursor: pointer;"></div>
<div id="student_iframe_container" style="left: 48.5px; position: absolute; top: 2px; z-index: 9999; opacity: 1; display: block;">
    <div class="b-close" style="padding: 8px; font-size: 14px; font-weight: bold; width: 52px; text-align: center; cursor: pointer; overflow: auto;">
        <img title="Close" src="/user/74/187888.png">
    </div>
    <iframe width="800" height="670" style="margin: 0px auto; border: medium none;" id="student_138871" src="/index.phtml?d=825090"></iframe>
</div>

The student_iframe_popup class is used in this segment of HTML:

<li>
    <a id="138929" class="student_iframe_popup" href="javascript:void(0);">Student Name</a>
    <div>23<img alt="Red Flags" src="/user/74/187894.png">&nbsp;0<img alt="Interventions" src="/user/74/187895.png"></div>
</li>

My Flags:

<div class="b-modal __b-popup1__" style="background-color: rgb(0, 0, 0); position: fixed; top: 0px; right: 0px; bottom: 0px; left: 0px; opacity: 0.7; z-index: 9998; cursor: pointer;"></div>
<div id="student_iframe_container" style="left: 0px; position: absolute; top: 2px; z-index: 9999; opacity: 1;">
    <div class="b-close" style="padding: 8px; font-size: 14px; font-weight: bold; width: 52px; text-align: center; cursor: pointer; overflow: auto;">
        <img title="Close" src="/user/74/187888.png">
    </div>
    <iframe width="800" height="670" style="margin: 0px auto; border: medium none;" id="student_138978" src="/index.phtml?d=825090"></iframe>
</div>

The student_iframe_popup class is used in this segment of HTML:

<tr class="open">
    <td class="student">
        <a id="138978" class="student_iframe_popup" href="javascript:void(0);">Student Name</a>
    </td>
    <td>2nd Oct 2013</td>
    <td>10:24am</td>
    <td>IT Misuse</td>
    <td>Behaviour Unit</td>
    <td>Registration</td>
    <td>Referred to tutor</td>
    <td id="1994"><select><option value="O">Pending</option><option value="C">Completed</option></select></td>
    <td rowspan="2" class="centre"><img title="Notify administrator of mistake made on Flag given to Joshua Nichol?" src="/user/74/187888.png" id="1994"></td>
</tr>

Associated CSS:

#container table { width: 100%; }
#container table img { width: 16px; }
#container th { text-align: left; border-bottom: 2px solid #999; padding-bottom: 2px; text-transform: uppercase; font-weight: normal; font-size: 10px; }
#container tr.open { background-color: #ffd6d6; }
#container tr.no_action { background-color: #ffead6; }
#container td { padding: 8px 0 4px 0; font-size: 12px; }
#container tr.border td {  border-bottom: 1px solid #999; padding: 4px 0 8px 0; font-size: 11px; }
.student { font-weight: bold; padding-left: 5px; }

Obviously the discrepancy lies with these lines:

Right...

<div id="student_iframe_container" style="left: 48.5px; position: absolute; top: 2px; z-index: 9999; opacity: 1; display: block;">

Wrong...

<div id="student_iframe_container" style="left: 0px; position: absolute; top: 2px; z-index: 9999; opacity: 1;">

However, it isn't me that's writing those styles - it's bPopup. Neither of my pages have any CSS styles relating to bPopup; the only ones I'm changing are those in the JavaScript file at the top of this post.

Does anyone know, therefore, why bPopup decides to left:0px on one page and left:48.5px on another?