Let me explain how I think.
Drawing an Event doesn't decrease the number of Kingdom Cards you draw. Having drawn a specific Kingdom card doesn't decrease the probability that you'll draw (or that you have already drawn) a specific Event. In short, they are what I suspect statisticians call indipendent events (lowecase).
How did I come to that number?
(10/totalcards+1) is another product of my spaghetti statistics. In a deck of 250 Kingdom cards, there are 251 slots where events can hide between real cards: 249 recesses between cards, plus the top and the bottom.
You'll visit 10 of these recesses when you're setting up the game, and include all the events you'll find there. The probability that a specific event is in one og those 10 recesses is of course 10/251.
Correct me if I'm wrong, please.
Edit: readability, mostly