I suspected that the intrinsic behaviour was different, thank you!
Interestingly enough however, it doesn't behave the same with Volume_Up and with Volume_Down.
With Volume_Up the volume steadily increases but with Volume_Down it starts normally then suddenly accelerates.
EDIT: This has actually nothing to do with autohotkey but is the behaviour of the Volume_Down key on my computer. After more than 6 fast presses, the volume step changes from 2 to about 10. Funny that this only happens with Volume_Down though. I guess it's better to be able to quickly turn the volume down than up