I guess you added to Favourites photos from different upload dates. When you view Favourites as list then you should see all those photos you just added, but when you view Favourites as gallery then it's sorted by newest uploaded to the site first, not by newly added to Favourites, so the other photos you added but can't find are probably somewhere in the depths of that Favourites gallery.
I did notify Hugo some earlier about Favorites to be sorted by newly added in there in both list & gallery modes.