ToME: the Tales of Maj'Eyal

Might be that the term in subject is wrong, but I refer to items that you can use to 'cast' a spell or the like. They sometimes have listed Power x / out of y, or something similar.

I've noticed that I can keep on using these items without running out of power in the item. It might cost 20/20, but after repeated usage, it is still 20/20. How does this really work?

Try looking at the item right after you use it.

I have... Still couldn't see any difference. Does it recharge or what?

A theory of mine: It uses my own mana, until I've got 0, and then it uses the items mana. Just a guess...

It's supposed to use the item's accumulated energy after which it'll restore by 1 per turn.
If that's not the case then you must have a bug in your hands. What items behave like this?

All I've used so far... gotta test more.