Light travels less than two thousand miles in 10 ms, and TCP requires three one-way trips before starting the first request on a new connection. Anybody more than 620 miles away (about half a time zone) is guaranteed to have a higher ping time than that.
You're absolutely right, I didn't think about that. However, I did perform the test(s) from Sacramento, CA to a server in Newark, New Jersey -- a distance of 2,810 miles according to Google.
I was fairly careful with the test(s), and the 10ms difference seemed to be consistent. So that's odd. I need to investigate that further.
