I don't think the answer to your question is known yet.

There are a few studies that do find elevated B6 levels, in some people who do not supplement. This suggests some genetic error in B6 utilization.

One study was accidentally found in autistic children, who were tested routinely before beginning B6 supplements. This study was to see if B6 helped their cognition and emotional response. What was unexpected was to find elevated B6 levels. They were not taking any supplements when tested.

Most people do not get tested for B6... so there is a huge unknown population out there who may reveal many problems in this area. What is puzzling is why this is not excreted by the kidney, which one would expect. I suspect some genetic error some where that causes this.

An example I found recently was regarding hereditary angioedema. One site suggested that family history for this genetic error, doesn't show up in some people. They suggested some kind of damage to the embryo during early development that damages the enzyme C-1 inhibitor, that removes bradykinin from the body. So that suggests an error during early development of the embryo may occur for other problems too, and those have not been discovered yet.

Do you know for sure that P5P was specifically tested for? My impressions of the B6 test are that it is not that specific.

Another possibility is factititious elevation due to handling by the lab or the blood sample. An example is potassium serum determinations, can be falsely elevated if the red cells burst (lyse) in the sample. I don't know if B6 in the red cells is high, since that is a specific thing only testing labs know IMO. But if your red cells burst and release B6, it could result in a false high reading. The cells can burst from too tight a tourniquet during the draw, or if a butterfly thin needle is used for small veins.
Usually the report will say Lysis of cells, somewhere on it, but not always. I just don't know if B6 can have this false high.

Labs seem to make mistakes also on nutrient testing. They don't seem to calibrate their machines carefully, etc. This happened twice in US with Quest labs, which reported false Vit D levels twice for at least a year each time!

I think you need to ask the lab itself about procedures etc. Often times doctors will re-run tests that are iffy.