Most Lexus dealers recommend pad replacement at 3mm. This is because at 2mm and under, the metal wear sensor can hit the rotor and cause damage that may or may nor be able to be fixed. As for the question as to why the rear brakes wore so fast is because most technicians use an eyeball measurement (primarily) of the pad thickness. What one technician would say is 6mm, another would say is 5mm. My guess is that you really had closer to 5mm the time before because as a pad gets thinner, it is easier to see what thickness is left. I would guess that you won't need to replace the pads for another 5000 miles.