I don't think anyone knows for sure what PMR is. It is "associated" with GCA which is a type of vasculitis. There is "large vessel vasculitis" or LVV. However, PMR might be something different ...possibly "small vessel vasculitis" which interrupts the blood supply to muscles and connective tissue.

The area of inflammation tends to be widespread and affects "many muscles" and that is how PMR gets its name. The area of inflammation isn't seen in the muscles though. The test you had (MyoMarker 3 Profile) I think tests for muscle inflammation which is called myositis.

PMR is not the same as myositis. The area that has inflammation as seen in PMR is in the synovium which is connective tissue that lines the inside of the joint capsule and is filled with fluid. That kind of inflammation is called synovitis. Maybe the following link explains it better than I can.
https://www.nature.com/articles/ncprheum0520#:~:text=Studies%20of%20patients%20with%20PMR,synovitis%20of%20the%20large%20joints.
The exact cause of PMR is unknown. The difficulty in making a diagnosis is that many types of inflammatory arthritis cause inflammation in the same area as PMR. The question most often is whether a patient has PMR or inflammatory arthritis. In my case ... my rheumatologist says I have both.