You have a very valid point.

Ballisitc gel can give an approximate measurement of penetration of comparibly high-velocity bullets where the human skin is negligible.

When it comes to much lower velocity projectiles like slingshots and spring loaded BB guns, the skin's resilience and elasticity plays a much larger role in getting an accurate reading.

But really all you have to do is shoot someone in the head with one of those marbles with a slingshot, and there's an excellent chance a skull fragment will chip off and end up in the target's brain.