Why does the tool show weird pings (0 or 999)?

[modder]

We do not have a mechanism to trace the real ping.

Instead, we calculate the average ping of all players. This gives
you a good overview of the gameplay you might expect on the
server.

This means, a server shows ping 0 if there are no players online.
On the other hand, a server can have 999 ping if all players have
999 there. This can happen if a new map is loading.