Different people use different dilutions and developers for printing. I've always used Dektol 1+2, but some prefer 1+3 -- and others like Ilford PQ Universal, Ilford Multigrade, Adox MQ Universal, homebrewed Selectol Soft, various warm tone developers, Ansco 130 -- there must be a hundred different print developers.
All of those will develop at different rates.
On top of that, different workers have different standards on when a print is "fully developed" -- for some, you want to develop to completion, doesn't matter if that's three minutes or ten; others will run an actual timer and pull the prints after 2 (or 3) minutes, and expose more (or use a higher filter grade) if they don't have the blacks they want, still others will watch the print and snatch it when it "looks right" (that's the way I learned to do it in 1969; I've since learned "better").
Ilford doubtless has some other recommendations that go with their "recommended" time of 3 minutes -- a particular developer at a particular dilution and temperature, at a minimum -- but give the wider range by way of saying "no matter what print developer or method you're using, you probably won't get the best prints with less than two minutes, or more than five."