If you look closely a lower case p is just a d rotated by 180 degrees, so the answer is obvious - there are a lot of rotationally dyslexic people around

That's IT!!!! 
I’ll guess the recommendation to have the image in PPI that divided with the paper-feeding direction number result in a whole number is probably to avoid problems similar to how a scanner works.
Recommended for Epson: 720 (or 360) ppi, for Canon printers: 600 (or 300) ppi.

