How exactly does Keymaster work?

The perk "Keymaster" (Pickpocket) states "Pickpocketing keys almost always works."

What does this mean? Does it mean that all keys will always have a 90% chance (the cap) to be pickpocketed?

If so, are there keys later on that are really hard to pickpocket? Because where I am currently (not very far) all keys I've come across already have a 90% chance to be stolen, so I'm wondering if this perk is worth it.

Solution 1:

Given that you already have 90% and it will be 90% afterwards, it's not worth at all to buy the perk.

You could just delay buying the perk until you see a chance lower than 90%...