I think the FB app can only access local files, so if your pictures are on Photos (cloud storage) you won't be able to see them from the FB app. You can, however, share from the Photos app into FB.
This is a design choice by the FB team. They chose to implement their own Gallery for in-app pictures. And yes, for some reason, the iPhone app works differently. You can still access your local and cloud files if you share the picture from FB's mobile site, though. It's just the app that's 'limited'.