Category lists in the home page are truncated to the first 16 from an alphabetical sort of all existing entries in each category (Instruments, Composers, Styles).
This results in categories with high representation in our repository to not appear, while prime real-estate is devoted to low-count entries:

• Instruments like Piano, Voice, Violin, not shown
• Composers like Bach, Mozart, Beethoven, Giuliani, not shown.

I believe we increase user value by selecting these short lists from the top 16 entries by piece count.

For the "Show All" link, I think it's safe to assume the user at that point is searching for a known entry, in which case it's appropriate to retain the current alphabetical order on the expanded list.