Another brake question

[Houston Ben]

I put on new pads and drums on the rear end of my truck not to long ago and thought they were doing ok. I am in the middle of installing a true trac and had to remove the drums and noticed the pads have worn smooth on a few spots and the rest of the pad still looks new, does anyone know what could cause this??

The pads and drums are brand new and shouldn't be wearing this way should they??

Thanks for any help!