... I've had one confirmed use of the new DRT ammo in 9mm, and it was a nasty but shallow arm wound that fractured the ulna but immediately stopped. It showed signs of fragmentation as soon as it broke the skin. It was definitely not a stop. Ribs are stronger, and the skull is extremely hard. (Some harder than others, witness Tat7...) They are also curved and designed (or evolved, as you prefer) to deflect damage away from the organs that are behind them. Bullets that hit straight on a flat surface of bone have a better chance of getting through, but the ribs do a surprisingly good job of routing stuff away from your giblets.
On a similar topic, I was extremely unimpressed with a couple of .38 shootings I witnessed.