> 2. Clicks with 100% bounce rate and that spent exactly zero seconds on your site. There is just no way these are real humans.
If you're using common analytics programs, you're probably misled about what the time-on-site statistic means. Unless they actively ping every visitor on your site the entire time your page is open, which is not the norm, they have no way to know the time-on-site for single page visits. It's computed as the elapsed time between two page views (two loads of the analytics script), but if there is no second page view, there's no second time to subtract from. Someone who clicks through to your page and reads intently for 13 seconds before closing the page is a "0 second visit" as far as those programs are concerned.
There's actually a legit problem here though where ad networks are counting "clicks" while analytics programs are counting "visits".
Comparing the two always leads to different numbers because they are different things.
Google has this issue even between Adwords stats and Google Analytics stats. They are always billing people for more clicks than are reflected as visits in Google Analytics. Same reason, different things counted differently.
If you're using common analytics programs, you're probably misled about what the time-on-site statistic means. Unless they actively ping every visitor on your site the entire time your page is open, which is not the norm, they have no way to know the time-on-site for single page visits. It's computed as the elapsed time between two page views (two loads of the analytics script), but if there is no second page view, there's no second time to subtract from. Someone who clicks through to your page and reads intently for 13 seconds before closing the page is a "0 second visit" as far as those programs are concerned.