That's because the tub is touching your studs, or possibly recessed into the studs.
Cementboard, waterproofing, thinset mortar, and tiles, can easily add 1-inch to each side, so your 60" already turns to a 58".
I'm having the same problem. I have a 1970s house. I measured just about 58" from drywall to drywall unless there is supposed to be more room behind drywall.
ThanksThere is more room behind the drywall.
The room is framed at 60-1/4" inches for a 60" tub. Then the drywall and tile goes in. Sometimes they shim the walls so that the backing goes over the tub lip. I have never seen anything but a 60" tub except for a custom made Bathfitter installation "over" the wallboard and tile.