Is there a reason for the discrepancy in the length of the Serpentine Road in the Japanese and English versions of Dragon Ball Z?
There does not appear to be an officially stated reason for changing 1,000,000 kilometers to 10,000 miles, and it is likely just another case of localization. For some, it may be easier to imagine distance in miles rather than kilometers, and would likely cause less confusion for American children who would be more familiar with miles. This distance is used for the Ocean dub, as well as the FUNimation dub.
Interestingly enough, this also differs for the dub for Dragon Ball Z Kai, which states that Snake Way is 1,000,000 miles long (or about 1.6 million kilometers).