Eye of Cthulhu
It's actually the other way around.you've got it backwards, unless expert mode did some weird stuff to skeleboi.
But Skeletron Prime loses defence per every hand killed.
Edit: I'm stupid and the screenshots didn't work. But go on the terraria wiki, it shows that Skeletron in Expert gets more defense per hands killed, while Skeletorn in Normal and Skeletron Prime in any difficulty don't. You'll also see this yourself if you look at the damage numbers you get against expert Skeletron or Skeletron Prime.