Clicky is much more accurate about determining the bounce rate. If someone views a page for longer than 30 seconds, it's considered a legit view. Unlike GA or Piwik, they ping the visitor every second that the page is in an active tab/window. So, you know how long someone is actually on your website.
Otherwise, like GA does, you could have someone leave a page up reading for 10 minutes or come back after getting coffee and then leave the site and it'd count as a bounce instead. Or worse, GA will count them as a returning visitor if they click on a different page after being idle for awhile instead of just continuing on with their session, which distorts your stats.