Open VisualEditor. Click 'Insert'. First item in the main Insert menu: Reference.
Open (or insert) and image. Under the Caption, click 'Insert'. First item: Language.
Reference is still the most important item, and should be listed first. (Should "image" even be listed at all? It works, if you only go one deep, but...)
Version: unspecified
Severity: trivial
So in general, the menus are supposed to be different for exactly the reason you hinted at: there are things you can't do in an image caption or in a reference. That said, the order should probably be consistent. This is likely to just be an oversight (the lists are maintained completely separately, which is kind of horrible) rather than anything intentional, but I'll double-check with James to make sure.
Change 117101 had a related patch set uploaded by Catrope:
Make the order of insert tools consistent in dialogs
Change 117101 merged by jenkins-bot:
Make the order of insert tools consistent in dialogs