A bunch of dice are rolled every time a war hammer connects. The chances of hitting someone to 10% with the best possible war hammer (does the newb not have strength potions?) are quite low, so let's not pretend like it's the norm. In fact, out of 84 recorded crushing blows against AR equal to invul leather, my highest damage was only 57. So, if there's a 25% chance for a crushing blow, and a 50% chance to land a hit (less if the rumors are true that Mark made defensive wrestling equate to 110% skill), that means that out of 672 swings of my hammer, I didn't get a single hit that would have brought someone even close to 10% health.
Before you're tempted to say "well, not everyone wears invul!" without realizing that even fewer people use a valorite war hammer, I'll also point out that even against a completely naked person, the average crushing blow damage was only about 68 over 63 crushing blows.
Things in the field differ a bit considering that armor breaks over time, but testing for that sort of thing is much more impractical. Hopefully they reveal the combat values and formulas at some point so we can figure these things out.