Hi Oliver,
Inbox shows those images that are not assigned to any album, while All Images shows all images in the library, regardless if they are or are not inside an album. See Help ▸ Observatory Help ▸ Using the Sidebar for more information. I just noticed a small mistake in there: "Some features of Observatory, like Stacking, are not available for images that still reside in the Inbox" is not true since Observatory 1.4.3. You can stack images directly in Inbox nowadays.
