I can reproduce this behaviour with explicitly shared pages (SP 2013 Standard)
- create testpage_1, check-in & publish
- break inheritance of pages library
- create testpage_2, check-in & publish
- share testpage_2 with permission_group
- reset inheritance of pages library
After this steps surprisingly the view group permission windows shows a permission for testpage_1 whereas testpage_2 is not listed!
Due to the resetted library permission we don't assume at first glance that there are explicit shares on pages.
So to me it seems that this view has problems to investigate the current permissions. And it handles explicitly shared files or pages differently than pages with inherited permissions.
If I'm asked whether we can trust the proposition of this window? Perhaps not (or not in this version I've tested it).
For the sake of completeness: in my test collections publishing feature is activated (not sure whether this lovely devil has side effects here too).