I’m pretty sure it’s just like how other types of onions (or other plants)have different color expressions.
There is a big stand of them in an easy near by to me spot that I go every year, and they are all lacking the red/purples on the stem.
Anecdotally, the ones I’ve found there also seem to have a higher water content/looser bulb structure, be a bit more fragile, and have a milder flavor.
Id be really curious to see if your experience is similar, but given you found it with all ones that have the coloration, I’m guessing maybe that one was situated someplace in the patch that whatever conditions trigger the color, didn’t happen for that one.
Either way, let us know if you come across more white ones and any other notable differences between them.