This one was kind of obvious, but took me a while to see though.
Ever had issues with setspn being very slow when listing or adding a new SPN ?
Did you notice its only when working with user accounts, but not machine accounts ?
Look at the help text
So SetSPN will assume your working with a computer account unless you specify other wise. IT will work yes, but be slow, so if you are working with a User Account remember to add -U