correlation and risk reduction - port mgmt

i know there is a simple explanation but i can’t seem to grasp it. lower correlation means greater diversificaton benefits: understood. however, the higer the average correlation, the fewer stocks it takes to achieve a specified risk reduction. This seems like a contradicton to me. can someone please clarify? thanks.

Check this out:,662057,662514#msg-662514

Kerry, thanks. Very helpful.