I may be wrong but I always thought silnylon is not water proof but pretty darn close. Any small amount of pressure will push water beads through silnyon which is why sometimes in hard rain you get misting under a silnylon tarp. It may not in your test but an oz or two is not enough pressure to go through.

Here is a good article on keeping your gear dry and info on silnylon stuff sacks.
Keeping your Critical Gear Dry