Claim slips aren't really the same. You could represent a claim slip (or any KI) with a single bit. If it's 0, you don't have the KI. If it's 1, you do.
Items represent more data (stats, description, etc.) and storage slips work by interpreting this extra data as a series of flags (1, he has the armor; 0, he doesn't have the armor).
So, like, your key item list could be:
Spoiler: show
And this set of bits (5 listed) would be sent from the server every time you try to open your KI menu so that the client knows what to display.
Items would be something more like
Spoiler: show
So each Storage Slip is kind of like an entire Key Item menu stored inside of an item. It's just a big array of flags for whether or not you have a given item. It's kind of an ideal solution because the amount of data between client and server stays the same (as if it was any other item).
I don't know why they refuse to expand it.