One of the definitions for 庫 is ‘kura’, which, to my knowledge and that of the Wiktionary community, isn’t an English word. Obviously there are some Japanese words that have been borrowed and can therefore be known by their Japanese names, but is this really one of them?
They give storehouse as the primary meaning. There are other kanji that mean storehouse and have the reading of “kura”. I think this is just a link between the other kanji meaning storehouse/kura. I try not to worry too too much about what goes on in the mind of the controllers of the wanikani universe.
I wouldn’t have noticed on WaniKani itself, but I go on to KaniWani after, and it’s not possible to ‘remember’ something when it gives you the answer…