Daystate airwolf mct.25

Could be many reasons....

hammer spring tension has decreased slightly as the spring has worn from being cycled and under compression.

probe seals could be leaking a bit

transfer port seals could be leaking a slight amount

maybe slightly larger or smaller pellet diameter causing that difference

Since you have the MCT, it could be your battery, capacitor, or something else electronic. 

Using the same chronograph??? Same distance? Same lighting???? 

Honestly, if the rifle still shoots well and doesn't have anything obviously wrong, I wouldn't worry too much. For peace of mind, send it to AOA and have em go through it and do a re fresh on the seals and check your electronics 

cheers. 
 
I dont think there is anything wrong with it. Too many variables to worry about 20fps. Hell....one tin of pellets to another could cause most of that. A slightly different angle on the cronograph could cause that. That is truly nothing to sweat. A dirty barrel could cause that..... or an infinite combination of the three. Shoot it and have fun.

This brings up a good point.....and something I personally do with a new rifle after a tin of pellets. Clean barrel. Set chrony up 10 feet in front of muzzle. Actually measure the distance. 10 feet can be 20fps depending on pellet. Level chrony with an actual level. Level barrel with said level. Shoot a shot string through chrony and hold the exact same spot for every shot. If the need arises down the road.....duplicate same process with same crony to get a more accurate comparison.