I used PhotoSweeper to look for duplicate photos in Photos. (Thank you ZacisBac.) It flagged nearly 70 pairs that were actually the same file name (so the same file, twice in Photos) that were NOT present in iPhoto. [There were actually another dozen or so similar pairs that were also in iPhoto.]
These photos were all in 3 dates so I did a side-by-side comparison of the pictures in Photos & iPhotos and came up with another bunch of images in Photos only.
It does appear as if iPhoto must not have actually deleted the photo file when it "deleted" it from the DB.
I've found 193 images to delete – and am down to 10,164 (from 10,357) in Photos. (iPhoto has 9,172)
I think the only way to find the others is to walk through both programs side-by-side and at 10,000+ images I guess I'll just re-delete the images in Photos as I come across them.
I suppose there may be other reasons Photos has more images but I'm just going say this is solved. Thank you léonie.