I noticed it when searching for a Fully Reviewed addon that I created recently. When searching on AMO, with a single keyword (refresh), it comes as as the 4th listed (both in the pop-up suggestions and the search result page) but when searching via the built-in addons search in about:addons, it doesn’t come up at all!!?
Are there some filtering involved (e.g. marketing, etc) or does about:addons use a different server (which makes no sense) or does it use different criteria (which makes no sense either)?
Assuming its ‘Refresh Blocker’ (Fully reviewed addon was a little vague) it’s 2nd place in about:addons and AMO for me. The only filtering should be platform/version filtering - i.e. about:addons is supposed to only show compatible add-ons with that version of Firefox, whereas AMO will show some incompatible versions too but greyed-out.