Someone please explain :)

Very simplified explanation (as much more is in play such as valve opening amount, time valve is open, port size of the valve and other variables): Your hammer spring has so much force available to open the valve when the hammer is released and hits the valve to open it. The valve however is closed by the force of the pressure in the tank (and by a valve spring as well in some cases). When the tank is at a high pressure the valve is not opened as long or as far by the hammer so a short burst of air is released behind the pellet. When the pressure drops in the tank then the tank valve opens more and closes slower due to the lower pressure and therefore releases more air behind the pellet.

Thurmond

It's the bell curve. Unregulated guns on a full fill start low fps, build to a high point, then start dropping. This happens because of the high tank pressure pushing back against the valve stem when the hammer strikes it. As tank pressure lowers, the hammer can open the valve easier. The drop is from the reservoir losing its pressure.