As I have been repeatedly reminded in this thread though, the DM should adjust encounter difficulty for things like surprise and ambushes.
So I have now changed my encounter from 2.1x deadly to 3.1x deadly (I guess we just increase x by 1 since we are already completely outside the bounds of the encounter system, who knows), and of course this assumes that the hill giants with a ....ehem....-1 stealth check are ambushing the party. Assuming every single party member has a 10 passive perception, the odds that all 3 giants will win stealth and the entire party is totally surprised is 9%.
So yeah at a 3x deadly we finally have a 9% chance to ambush the party and maybe kill the wizard.... 3x deadly. And hence this is where I get such numbers in my game. I find deadly x2 is a "reasonable encounter", and deadly x3 is where things are actually tricky and my players might get scared. It takes a LOT of stuff to maintain 2x deadly encounters.