When my Unraid system used reiserfs as the array file system  I almost never got a parity error correction. Now since I have converted everything to xfs I notice one or two hundred errors being corrected each month when the scheduled parity check runs. Why is that?

Parity check knows nothing about the file systems as it works at the raw sector level so any issues are unlikely to be related to the fact you changed file systems.

Start by running memtest.

Running memtest did identify a problem. Thanks for the suggestion for both my posts about issues I was having. I think the BIOS version of my motherboard was the root problem, particularly since the memory was being overclocked.