iTunes alphabetising 'Z' above 'Aa'

In my iTunes the artist 'Aaron Copland' has been demoted to the bottom of the Artist column, below artists beginning with 'Z' (eg ZZTop, Zoltan Kodaly etc.). Similarly in my iTunes Library on the Mac Aaron Copland (spelt exactly that way, Capital A followed by lower-case a) appears below the Z's.

It's not a huge problem but it's annoying and I don't understand why it's happening. I can't figure out how to change it so it follows proper Alphabetical order. It must be something Apple decides about a double A, since every other artist beginning with 'A' is at the top of the list. I've managed to get Aaron back to the top of the Artist column by going into the 'sort as' file and sorting him as 'Ab', but he still is residing at the foot of my iTunes Library.

Any ideas? and why is this happening? it's only begun recently and I haven't updated iTunes in years. The same files for Aaron Copland have been there all along.