I've only got about 1.2 miles of two strands. My meter only goes up to 5000 volts and that's where it's registering. I guess your fence is just hotter.
TNcattle, 5000 volts will keep them in after they are used to it. That's about as much as I ever have. I had a cheap meter that read up to 5500 V and I hooked it to a new charger that had more voltage and it fried it the first time I tested it.