While changing the pads for my rear breaks (disk), I saw that while the external pads were all gone, the internal ones were at 1/3 to 1/2 only.
Is this normal? Should I address this? What is the cause - for self educational purposes?
They are supposed to wear evenly. The most common cause that I've seen of uneven wear is the caliper slides sticking. Often, the slide bolt boots don't seal real well, allowing water to get in and wash the grease out. Once that happens, they start to stick, causing uneven brake wear. In my experience, it can also be a problem if the car spends a lot of time sitting (even if the boots appear to be OK).
Check your alignment. If the car has ever been hit hard, the car might be driving down the road "sideways", that is that the back tires are not pointed straight. That will lead to uneven tire wear, uneven rear brake wear, handling issues, among others.