the reg on your bottle will be set to a determined pressure (800psi) but a cheap reg will deliver 750-850 psi as an example and this output pressure powers your gun.
if you set the velocity on your gun to 280 feet per second while the bottle reg happens to be delivering 780 psi means if the reg pressure spikes to 840 psi your guns velocity will increase. you need to take this into account when setting up your gun and allow for the pressure increase by turning the velocity down a touch, until you are used to how your gun performs with that bottle you will need to check your velocity often to ensure you are still on or below the site limit.