Anycast proxies are a great protection mechanism against DoS attacks, but they significantly affect the ping reported in the server browser. For example, a server hosted in Australia may have a ping of 40ms for players in the region but a ping of 300ms for players in Europe. Using an anycast proxy, in this case, would report a much lower ping (e.g., ~30ms) to players around the globe and incorrectly sort the server among those with the lowest ping.
This is frustrating for players looking for low-latency servers, as they may join one with a low ping only to find it is much higher in-game. Servers using an anycast proxy are flagged to sort like they have a slightly higher ping to avoid this problem. It was implemented as a direct response to complaints from players.
Servers using a regular proxy with ping similar to the actual in-game ping are not flagged. Only proxies with a significant ping difference are flagged.