Assumption: You track via Piwik in a page that is included via an iframe
By default, as soon as the focus of the browser tab is lost (by moving to another tab or any other window) we stop sending ping requests. However, this does not work if someone is using Piwik in a page that is included via an iframe. In this case we keep sending ping requests for as long as the browser tab is open.
Note: Not sure if we can actually fix this because of cross domain policy.