One user may click 5 ads in 1 minute, and all those clicks may be treated as good. Example: you land on page which have a big block adsense unit (4-5 ads in one unit). If ads are on topic, visitor could easily click all the ads to look what is behind them. Same for link unit. After link unit is clicked user is presented with a list of ads. Why do not click 4 or 5 ads if they are on topic?
I believe number of clicked ads is only one (and maybe not the most important) of the NNN factors which identify invalid click.
BTW, clicks can also be treated as invalid if you place ads very close to right-hand scroll bar, close to menu (i.e. when user clicks them by mistake, trying to perform another action), or if your traffic is suspicious. And if you get too many invalid clicks you'll either receive a warning email from Google, or site (or maybe account) will get banned.