ITunes does some bloody weird things with compilations - or what it perceives of as being compilations. Personally, I think a CD collecting the 'Best of' a single artist should be catalogued under that artist.
Not only does iTunes randomly decide that some collections are compilations, rather than albums by that artist, but this also means that when an artist appears on a compilation they don't appear in the list of artists on an iPod. To find if you have an odd track by, say Tallulah Gosh, you have to do a general search because scrolling the artist list isn't enough.
So,for the Alphabeticon, if artist releases a collection of their own work, they're going to appear in the A-Z lists of artists CDs (as will collections of covers).
Additionally, if I have odd tracks by certain bands, they're appear in the appropriate letter as 'random tracks' (e.g. The 'A' List - random tracks) with a cross-reference from the main CD entry if I also own a CD by them.
Certain compilations will be treated as themed collections under genre (e.g. Pillow and Prayers, the Cherry Red collection, will appear under Indie collections).
It's going to be confusing but hopefully complete!