The rear width is effected by so many things, such as stiffness of the frame, the vertical and fore-to-aft location of the seat, and the size of the driver, so it’s almost a given that two drivers would find themselves tuning the rear track to different widths.
Why the concern? How is the kart performing against it’s competition? If you are competitive at that width, does it really matter?

Older karts don’t function at 55″ usually. Track widths used to be narrower for most karts. Not only that, but a 13 year old kart probably won’t ever function the same as a new kart because of it’s age.

While it’s often true that a very narrow kart will not be as stable, and, as an aside, a SUPER narrow kart will just bog the engine down, that doesn’t mean that it’s not drivable. Though it’s advisable when driving a new kart, to start the rear width near or at the max width, you ultimately need to put it where it’s going to go fast whether or not that’s at 55″ or 53″.