Our neighbors had an Australian Cattle dog, he was a mean biter. He especially like to bite runners in the calf as they ran past his house. They reminded him of cows I guess and his instinct was to 'heel' them. Some dogs are guided by instinct more than training.
I have an Australian Cattle Dog and they ARE instinctively programmed to chase moving things. Mine only lightly nips anything he's trying to herd, but I've yet to be able to stop him from chasing cars.
If you're not running, he's not the slightest bit interested in nipping you.