A shot in the dark. Could it represent manufacturing variables? Regardless of the level of precision there are some variables. The weight of each locomotive is different even if the drive ratio and the motor are the same. An F3's weight is different than a GP7 or Gp-38 and just like paint, motors with the same specs on paper can very from one production run to another. The most common variable is the flux density of the magnets. As the flux increases from weak to strong on a typical motor there are two sweet spots one for rpm and one for torque. I bet on a well designed motor those points are much closer together on the curve. Flux density is less variable today than back when I was rewinding slot car motors in the sixties. Back then you could find two "identical" new motors where without a pinion gear your fingers could barely spin one over the cog and one you could spin like a top. The one with stronger magnets would always have more torque, run cooler and pull taller gearing. Racing and dynamic braking would eventually weaken those strong magnets and for a buck or so most slot car tracks could re-magnetize your magnets. You do reach a theoretical max flux density where an iron core armature cannot turn at all before the commutator and windings fry. Fairly sure that does not occur with coreless motors. Back to mine and Teds hot button, as the gear ratio goes up variables in the motor have less effect. And then there is this. Back when Lionel made the 18000 PRR B6 I was not at all happy with the slow speed performance of mine so I dug through my box of slot car gears till I found a pinion and spur that would properly mate with the given fixed space between the motor shaft and the shaft the worm was on. I cannot remember what the factory ratio was and what I came up with it was only a small increase in ratio. If 1:2 was stock my gears only made it something like 1:2.5~2.6. Anyway the final drive ratio went from about 14:1 to about 22:1. You would expect the slowest speed would have been slower. NOT with the ZW which has a low voltage setting no lower than about 6v. Without a load the loco was faster at 6v than before I changed the gear ratio. However when pulling a load it would go noticeably slower before stalling. Had you been able to dial the ZW down to one or even three volts the loco's slowest speed before stalling would have been slower. Shows the importance of the power supply in the complete picture. What I did in response to the increase in speed without a load was to put several diode pairs on the pickup rollers. Which may have accomplished what I set out to do without the PITA that changing the gears was.
I'm sure if you could test enough MTH locos you would see a pattern but simple manufacturing variables could account for the difference between the 0x08 vs the 0x09. j