[Vince]

I believe traditional ping requests, which rely on ICMP, only display the "pong" answer, i.e. the time it took for the answer from the server to travel back to the client, whereas the in-game ping probably displays the time it took for the request to travel to the server and back ("ping-pong"), effectively doubling the displayed ping time.

I might be wrong though. I have no idea how the internals work and I can't be bothered to do an analysis right now.