[smartass mode: on]ping != lag[smartass mode: off]
I experience the same. My ping, usually constantly around 220-240, has spikes that seem not to be caused by MY crappy ISP/line.
And there are hefty lags, esp. on some custom maps. could be that those maps are bit overly complicated or have bugs and get laggy on slower computers.
Could be that the server is operating at his capacity limit. With 10-30 players I rarely experience any lags on NA siege, if there are more, things get mental...