That's usually related to using the wrong kind of volumizer on the 2k5 block, the 2k2 volly on a 2k5 block will make the marker finicky at best and a nightmare to tune at worst. Using the proper, shorter, 2k5 volumizer will help lower HPR pressure while maintaining the marker's handling characteristics. General consensus is however that 2k5 blocks don't benefit much from a volly because they are already higher volume.
If you're running a 2k2 block, then it's highly recommendable to get a volly.
Higher volume allows for lower pressure, because you're essentially getting the same amount of energy from a larger volume of lower pressure air (you don't need fluid dynamics to understand this, a quick introduction to gas physics or thermodynamics should be good enough
). What it boils down to is the slightly larger amount of air can move slower and still provide the energy to get the paintball going at 280+ fps, this results in a quieter shot that is more gentle on paint and because the poppet is still only open for the same amount of time, the HP chamber in front of the valve doesn't loose as much pressure, resulting in faster recharge.
You can refer to this explanation by timmy (& now shocker) guru Lurker27: