For the moment sets can store 499 items, since it considers set itself as one of the items.
It's a simple matter of 1 item, but when storing big amounts of items it is much more convenient to make it a round number, which is 500.

It's most likely a minute or two of work for a qualified staff member and it would make it much more convenient to store items for everyone.
Again, before anyone comments "but it won't change anything to majority of players", no, it won't, but generally speaking it would make much more sense and all it takes is couple of minutes.