Alexa does have biased data, so from that test the people who have the alexa toolbar installed go to sitepoint a lot too
It's not all that mysterious if you stop to think about it. Sitepoint and DP attract pretty much the exact same audience demographically. So a bank holiday dropping traffic here will drop traffic there. A major sports event keeping DPers away from DP will also keep SPers away from SP. A big news item about Google or a PR update will cause an increase here as well as an increase there.