Read Why Are Headed Goals Are More Difficult To Save In Football

Having watched over a 1000 football matches, I've come to notice that headers are almost impossible to save.

I've regularly seen keepers save leg shots from point blank range, but once a striker gets a header on target, 9 times out of 10 it ends in the back of the net, even if it's from some yards out.
So why is it difficult for keepers to save headers? And yes I've checked Google but didn't get satisfactory answers.