Tightening a nut to 100Nm that has been greased will equate to around 120Nm of pressure on the threads from a non greased fastening. It's basic physics, it will present less friction and therefore turn more before it hits a specific torque.
A simple demonstration would be to clean the threads of both axle and nut with something like brake cleaner, so it's oil free, then tighten to, say 60Nm (the figure itself is unimportant). Then mark the nut and swing arm position relation, then undo and cover the threads in grease and repeat, the nut will continue to spin past the mark before 60Nm is reached, thereby actually putting more pressure through the bolt (axle) for the same torque. To counter this we drop the torque setting.