The autopilot has limited aileron authority compared to manual input. The following AAIB report states:
Enquiries of the manufacturer revealed that the maximum roll control deflection which the autopilot can achieve is, by design, about 4.5° of aileron movement (a control wheel movement of about 20°), depending on the stimulus. The maximum aileron movement which can be achieved by the pilot is about 20°, which requires a control wheel input of about 82°.
AAIB Bulletin No: 2/97 Ref: EW/C96/9/3 Category: 1.1