I spent ten years working in mental health in a non-medical role.

They look exactly like the self cutting scars I have seen. Upper arms and upper thighs were the most common. Usually on the left arm, so they can use the right (dominant) hand to cut.

Scars like that are from cutting through the skin layers to the fat layer underneath, then healing without stitches. That is why they are wider in the middle than at the ends.