This question comes up all the time. I think for most people it is a red herring.

If blood tests are done in labs that don't do this test often, then the calibration of the machines may be off. Also there are many machines and they they differ when reporting and use different ranges.

Blood cells can lyse ( burst) when handled and this would release the contents of the cells into the serum specimen. This is well known with potassium readings and many of those are factitiously high (artificially high) also.

The B6 in supplements and food are not all pyridoxal in form, and pyridoxine(non active form) may not be converted well in some people (failure in pyridoxal kinase enzyme), and may build up in the body and show up higher in testing.

The actual numbers of patients damaged by high B6 took over 100mg a day for a LONG time to develop the toxic B6 syndrome. Many used 500mg a day for years, to treat PMS, carpal tunnel and other things by doctors.

I have yet to see any study showing elevated serum levels which gives the actual amounts of B6 those patients developed symptoms from.