That would a case of people doing this to counter volatile prices if people also took advantage of the quite low gas prices they chance upon to fill it their car up completely.

But what the article says is that they don't put more the $20 at a time, period.

Such a behavior wouldn't see any advantage given volatile prices, because in the end you end up paying the same (you pay lower at some times, higher at other).

Which is the article points too: that they see the guys doing that end up paying the same amounts as the guys filling it up normally.

So it's not the volatile prices that get people doing that, it's perhaps a false idea of saving money, or that they infact do not have that much lying around and try to get to the end of week accounting for any emergency.