Yes this is "normal", but is something you need to keep an eye on. SEDELUXE05's explanation is correct. But another reason, which is very common and an easy fix, is that the brake pads aren't sliding very well inside the caliper. So, for example, when both pistons go to push the pad. If the top part of the pad slides easier then the bottom, over time the pad will begin to wear unevenly because the top part of the pad will connect with the rotor first (and ultimately a longer amount of time).
So check the caliper slider pins (usually covered by a rubber, accordion style boot/tube), and clean/grease if necessary. Also check the surfaces of the caliper that the pad will slide against. Make sure they aren't dirty/rusty/pitted. Clean them off as well as you can. I like to put a little dab of all purpose grease on those surfaces (note: NOT on the pad or the rotor, ONLY on the surfaces on the caliper that the pad slides against) to ease in the pads movement in the caliper. If you get any grease on the rotor or pad, make sure to clean it off with some brake cleaner before going for a test drive.
Good luck...