I believe you. That's what I thought, too, until I saw this. The finished alcove wall is 57-1/2 in wide. Tub enclosure sticks out beyond that. From all outward appearances and measurements, it would appear the shower surround was made narrower than a typical 60 in unit. I have removed the sheetrock on top of the back of the surround and it is 1/2 regular indoor sheetrock. Having done that, I can tell you that nothing, and I mean nothing in this alcove, was framed with any rhyme or reason, or adherence to standard framing practices. Have a neighbor in the same housing tract whose finished alcove I measured and it was 59 in wide on the inside like it should be. They claim they had the same type of shower as this. And they have a newer cast iron tub installed. Makes no sense.
The concern is that there is only one bathroom in this house. I don't want to tear out the tub and enclosure, only to find that the alcove width is too narrow for a standard 60 in tub.