The part that varies house to house is the opening itself. A 1976 build was framed for the garage door widths standard at the time, and those don't always match the stock sizes sold today - sometimes by an inch, sometimes enough that the track needs custom brackets or the jamb needs a shim strip to take a modern door square. We won't know which until we're standing in the opening with a tape measure, which is why we don't quote sight unseen.
Headroom is the other one. Older garages sometimes have less clearance above the opening than current torsion spring setups want, especially if there's a low header or ductwork run across the ceiling. That can mean a low-headroom track kit instead of the standard one, which costs more and takes a bit longer to source.
None of this is unusual for a house this age - it's just not something a phone quote can catch. The measure visit exists specifically to catch it before the door's ordered, not after.