This is an annomoly. You get similar situations with some area codes crossing state lines in isolated cases. As the previous poster said this has to do with this town is a border town that is highly reliant on the people of the other border to provide things such as phone service.

There are some other geographic annomolies.

For example there is at least one county in California who gets a big energy break because its electricity source is Nevada and thus pays Nevada rates and not California controlled rates.