The British monarch has all those powers on paper. They never agreed to relinquish them. Then, random commoners (with no authority) keep asserting that the monarch has no powers, or no intention to use/abuse those powers. Nobody seems to agree which ones are not supposed to be used ever, which ones are supposed to be used sparingly, and which ones are OK to use recreationally. Then, occasionally somebody discovers some secret privilege the monarch has, and the people have to decide on the spot whether that's OK or not.

Compared with the actual constitutional monarchy, where there are actual rules spelled out clearly. The Brits are masters of wordplay, but "unwritten constitution" is really unironically "trust us, we're making this up as we go along".