From the forums: http://www.cegui.org.uk/phpBB2/viewtopic.php?t=3212
Alright i have noticed that if you are using injectMouseMove() with cegui your MouseEventArgs have a valid absolute and delta positions, however if you use injectMousePosition() it sets the position and then itself calls injectMouseMove(0, 0). This results in the MouseEventArgs for mouse move events not having a valid delta. It is infact 0 for X and Y.
I found this behavior strange and was curious why this design was chosen as it seem to make the mouse delta argument completely redundant purely based on your chosen way of injecting input.
Reproducibility: have not tried