As I said in the original post, there is a sliding scale above each bracket for the next $50k where you move up to full marginal tax at the next bracket. That’s why it needs to be complicated, because within that $50k it isn’t so simple.

Also where are you getting the effective rate from? The W2 doesn’t show what you actually owe, just what was withheld, so who is telling you?

The numbers appear as magic, but they aren’t, they are calculated exactly to achieve this effect. I know because I checked explicitly.