I'm using Facebook Graph API v2.2 (had this problem on v2.1 as well) and I'm requesting page_impressions through the insights endpoint for a set date range:
01-02-2014 - 28-02-2014.
After requesting I print out the impressions from the results, and most of the time Facebook returns a total of 6763 impressions. Sometimes though, a slightly different amount is returned but there doesn't seem to be a pattern. I've requested the impressions 40 times. The first 11 times: 6763, then once: 6709, 5 times: 6763, 1 time: 6720 and 12 times: 6763.
Is anyone familiar with this or having the same problem?
Thanks!
EDIT: I just had a result of 248 impressions... nowhere near 6763...
Seems this is a Facebook Bug. Found a bugreport about it on the Facebook Developers forum. Different endpoint, same problem: https://developers.facebook.com/bugs/877357682326329/