Easy, now I think I understand what you mean...
I was going to say that it might be because your joystick trigger is using a DX function to make the program fire the guns... but then I remembered that my own setup has the trigger setup as the backspace key in both the X36 SGE setup and in Il2's command list...
Then I realised that the I16 has a manual gear operation, you can lower the landing gear and raise the landing gear by a keypress... but it is manual so you HAVE to repeatedly press the manual gear down button until the gear is fully deployed and locked (green light on dash board).
So this leads me to believe that in the case of the trim by using a keypress, it is configured within Il2's own program that you HAVE to click the key down repeatedly in order to move the trim significantly.
This is only a guess of course.
-SW