The sum_nb_uniq_daily_visitors is unreliable, but it appears that you really found a bug as well!
Can you reproduce this on the demo.piwik.org ?
if you can, please post a ticket bug at: dev.piwik.org with API links showing the error. we will definitely fix it!
Can you reproduce this on the demo.piwik.org ?
if you can, please post a ticket bug at: dev.piwik.org with API links showing the error. we will definitely fix it!