Uneven Brake Pad wear? Causes?

With the wonderful weather we had this past weekend, I swapped my winters out and decided to measured the brake pad (103,000km on them) while I was at it..

Front-Right = 5.8mm
Front-Left = 3.8mm

Rear-Right = 7mm
Rear-Left = 8mm

I'm wondering what's causing the uneven wear? I mean... I got the brakes services by the dealership at 30,000km, 60,000km, 90,000km. Shouldn't the left and right be close to matching? The rear and front difference I can understand.

Please educate me.

Your calipers might be going

The slider pins probably need to be lubed. Dealer probably did not do this. Is that the total measurement of both pads?