Old Man Winter and Lower Numbered Cars...
It's pretty well-established that the GNX's that were produced before the spring thaw of 1987 tend to have more undercarriage surface rust than later examples. This is simply the manifestation of being exposed to harsher seasonal elements during transport, queue, and final delivery.
Although there are variations in build order (and assigned numbers), this is supported by details provided by owners regarding condition, with even zero or low-mile cars being delivered new with undersurface oxidation. And this effect is strongly skewed to earlier cars.
Of course, this does not preclude improper storage of a higher number car, which is often owner-induced (untreated concrete, moist/damp garages, etc.). But comparing apples to apples (trailer queen to trailer queen if you will), a lower number car may not present as pristine an undercarriage as a higher number with a later build date, especially when it comes to light surface rust and other small inequities (wire label wear, sticker deterioration, etc.).
Of course there are many other factors, so your mileage may vary!
Doug