Tubs are either 60" or 54" or 66" Are you measuring the tub after it has been pulled out? Or are you measuring from wall to wall?
Most bathrooms are framed at 60-1/4", and then you add drywall and tile. If you have a mud set tile job, then even with a standard five foot tub, the wall to wall distance can be 57". Somewhere buried in all that is a 60" tub.