When the droid rebuilds itself, it picks up a circular saw. When it uses that circular saw as a weapon later, it is completely different design, with an all different cutting disk.
After Jill attacks it with the buzz-saw, the Mark 13 steps back: if you look closely, you can see the legs of the man underneath the droid suit. The same thing can be seen towards the end as Jill and the robot are entering the bathroom.