Here's a drawing that shows the inclination of the lift angles in opposite directions as the wing rolls http://images-mediawiki-sites.theful...2414313322.png

An offset roll axis is an axis that is not along the middle of the fuselage for example it is slightly out to one side often due to an unequal change in the lift of up and downgoing ailerons. So the aircraft does not roll along the centre of its fuz, it rolls around an axis that is offset and possibly even outside the fuz. This gives it a slightly barrelly appearance to the roll and many model fliers wrongly call this adverse yaw. Making the aileron travels unequal can shift the axis back towards the centre and make the roll more axial and less barrelly. Aileron differential has corrected it, but what it has corrected is not adverse yaw!