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.