[STUPID QUESTION] i recently had 405 points, even though the limit is 400, how is that even possible

You probably won a game which you entered for multiple giveaways and those points will be refunded to you even if you had a total of 400.

You may also have removed an entry to a game which took you over the 400 point threshold.

Someone deleted a giveaway you entered.

Which is one of the ways I sometimes stock up points for GAs

There is no limit on the number of points you can have on your account, but you will not gain the 6 points every 15 minutes once you have at least 400. If you remove yourself from all the giveaways you are entered in, you could have a few thousand points on your account. You get points back from removing entries, winning and automatically having your entries removed from other giveaways of the same game, and when someone deletes a giveaway that you were entered in.

you got 0,125% interest rate on your points deposit ^^

View attached image.

A simple case of quantum entanglement.