The reason a 30" draw is faster than a 28" for the same poundage is you have increased the potential energy stored in the bow by crawing 2 more inches.
At the release this potential energy is converted to kinetic energy and genreally more arrow speed.

This is gernerally why the guys with arms like gorillas will always be shooting faster than those with T-Rex like arms.

It is the equivalent to using a larger powder charge in a firearm.