Ancalagon
Dusty Dragon
I'd say more like 4. 5 hill giants is 18000 xp, 4 is 14,400 (so within rounding range).
I think the thing to remember here is that was not actually a deadly encounter (and no I don't mean hard to win). For this party, this was 2.1x deadly.
Aka this is TWICE as deadly as what a normal deadly encounter is supposed to be. If 15000 xp was the daily budget, that means that the party should be dragging ready for a long rest after just one more of these (as that would be 5k over their daily budget)....and it sounds like they are still fairly fresh.
But at the end of the day this is nothing new. We all have our battlelines drawn as to whether you think 5e encounter design is well balance or not, and we aren't going to change any minds with this example.
Thank you.
And yes, I knew already that the encounter building system was not great, but I never bothered with it as a GM. I hadn't realized how bad it was.