There is a choice of rules when you set up the race, this year is the same as last year. Another option is every visit counts for the mileage, the way we do it encourages people to move it on sooner.
In the case of a TB that is dipped in a local cache and then gets taken to Australia by the same cacher then, as you say, the only mileage recorded is the local mileage. Bummer, isn't it.
We had a TB last year that went to Peru and Canada with the same cacher after bring dipped locally. It got a recorded mileage of 1 mile. :-)