Arduplane 2.68 on APM 2.5 motor spins when safe


I am using an old version of ardu plane (2.68) on and old APM 2.5 board. For some reason when SAFE, if i switch to FBWA the motor will spin up.
It will only spin up a fraction, not enough to move the aircraft. If i apply throttle input there is no change.

I put the latest version the board can handle on, as its retired, and this issue is resloved and does not happen.

I am just curious as the why this would be happening on the old version?
I know the fix is to update firmware but Im interested in why this is ocurring.

Any info would be great.