WMHelp XMLPad reliability?

Why would WMHelp XMLPad fail to validate 3, 5, 7 elements for an XSD that constraints element cardinality via minOccurs="2" and maxOccurs="unbounded" ?

I have this simple XML:

<?xml version="1.0" encoding="UTF-8"?>
<root xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="mySchema.xsd">
  <items>
    <item id="1">
      <levels>
        <level>one</level>
      </levels>
    </item>
    <item id="2">
      <levels>
        <level>one</level>
        <level>two</level>
      </levels>
    </item>
    <item id="3">
      <levels>
        <level>one</level>
        <level>two</level>
        <level>three</level>
      </levels>
    </item>
    <item id="4">
      <levels>
        <level>one</level>
        <level>two</level>
        <level>three</level>
        <level>four</level>
      </levels>
    </item>
    <item id="5">
      <levels>
        <level>one</level>
        <level>two</level>
        <level>three</level>
        <level>four</level>
        <level>five</level>
      </levels>
    </item>
  </items>
</root>

I want each <levels> list to contain at least 2 <level> elements. So here is the XSD I wrote:

<?xml version="1.0" encoding="utf-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:complexType name="ItemType">
    <xs:sequence>
      <xs:element name="levels">
        <xs:complexType>
          <xs:sequence>
            <xs:element name="level" type="xs:string" minOccurs="2" maxOccurs="unbounded"/>
          </xs:sequence>
        </xs:complexType>
      </xs:element>
    </xs:sequence>
    <xs:attribute name="id" type="xs:string"/>
  </xs:complexType>

  <xs:element name="root">
    <xs:complexType>
      <xs:all>
        <xs:element name="items" minOccurs="0">
          <xs:complexType>
            <xs:sequence>
              <xs:element name="item" type="ItemType" minOccurs="1" maxOccurs="unbounded"/>
            </xs:sequence>
          </xs:complexType>
        </xs:element>
      </xs:all>
    </xs:complexType>
  </xs:element>
</xs:schema>

When I validate my XML against my XSD in a tool called "WMHelp XMLPad", I get a warning for <item id="1">, but also - unexpectedly! - for <item id="3"> and <item id="5">. It looks like even counts of <item> (2, 4) are OK, but odd ones (1, 3, 5) are rejected.

Element levels {} has invalid structure for schema definition: (level{2..unbounded})

What might I be doing wrong?

That's an odd problem.

Neither I nor my Xerces-based XSD validator see anything wrong with your expectation: Only the number of <item id="1"> level elements violates the XSD:

[Error] try.xml:7:16: cvc-complex-type.2.4.i: The content of element 'levels' is not complete. 'level' is expected to occur a minimum of '2' times. One more instance is required to satisfy this constraint.

There are no other validation errors.

This appears to be a bug in WMHelp XMLPad.

Follow-up: The original domain (www.wmhelp.com, per this 2006 announcement) for WMHelp XMLPad is redirecting to sketchy sites, so it's unclear how one would report a bug.

Conclusion: WMHelp XMLPad is unreliable, unsupported, and should be avoided.