APM : lack of servo precision... Why?

Hello everybody,
First i would like to grant every develloper of these great Autopilot ! best job i’ve seems for life on a 8bit ( for Apm ) tiny cpu.
I got a strange problem, my setup :
Apm 2.5
Taranis 9xs
Frsky D4rII in CPPM mode

I got a lack of precision on my servo in manual mode compared to plugged directly on RX or with a servo tester.
The behavior is strange, it like the Pwm “jump” when i make some quick moves, and the resolution seems to be lower.
When using a digital servo the lack of resolution seems to be lower but its there.

The test protocol have been made on an apm “on the bench” so no interferences or power supply problem.
This behavior is not a problem on a bixler like plane because the command authority are often sluggish, but on a little wing like the caipirinha you can feel it very hard because of the great elevon authority. The feeling is the same when using a low quality TX.
The behavior is the same when i’m using the Cppm mode or Pwm output from the RX or a servo tester.

Is it a problem of pwm output frequency ? i don’t see any parameter for that on Arduplane.

Thanks in advance.

If you want i could make a youtube capture of the problem

I have the exact same scenario.
Any input or ideas on this subject?