Berkson's Paradox would be one potential reason. Say you have some criteria like "potential future donor" and "really smart". They might not be connected in th

Berkson's Paradox would be one potential reason. Say you have some criteria like "potential future donor" and "really smart". They might not be connected in the general population, but when you select on the total donor+smart, such that only people with a high combined score are admitted, you get a correlation among the admitted population. It's not hard to see why: either you have loads of money and thus only need a little smarts, or you have lots of smart and need minimal money.