The missing context (which I skipped) was that they had two identical(ish, I couldn't tell the difference, they were the same model though) cars, and the other

The missing context (which I skipped) was that they had two identical(ish, I couldn't tell the difference, they were the same model though) cars, and the other one did have that behaviour (having witnessed it), I suspect the cause was confusion about which car the different members of the family were using that day. It was also on the way back (so no-one was that stressed about it), so waiting to get back to Sydney would have given much cheaper petrol (NRMA came and gave us enough to get back out of the mountains after 30m).