I made some tests and it seems it comes from another part of xserver-xorg but I still don't know which one (evdev or the core xorg ?). I purged synaptics and the problem still happen with the additional mouse (package xserver-xorg- input-mouse is still installed).
I forgot to mention that even if I let the pointer in the middle of the screen, it slightly move of a few pixels like what happen when there is a lack of sensibility on an optic mouse. Hope this help.
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