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See report in http://forum.piwik.org/index.php?showtopic=7091
I think it is because for pages without a follow up page view, we don't know how long they spent time on, and we set 0 by default.
We should probably set the sum_time_spent for 10s for each page view for these pages (and have this as a setting in the config file)
(In ) Fixes #1259 Always setting sum_time_spent for pages in the API results. Also now setting time on site / time on page at 0 second (industry standard).