I can't understand why that happens.

Try making some changes to these three settings:

AP_Roll_speed=3.80
AP_Roll_window=7
AP_Roll_controlstep=1500

in the [Custom Autopilot] section. I pretty sure that others were changing them when they have that problem. It would be worth searching the 767 Captain - General forum for them. I never needed them, so I don't know exactly what they were changed to, nor where to find them. Sorry. :(