I would like to BS my way through the reason, but.....
....the end of the fretboard suffered, I suspect, from a banana sanding stroke by yours truly . I usually am pretty good at this particular aspect but, whatever, the zero fret was a tad lower than the others, even after levelling. I could have put an extra-high fret in there, but I still would need a nut for lateral guidance and it was just as easy to fix a nut. Pete didn't even know that he might have got a zero fret so no harm done there