kicker 06 s15l7 d2

I just bought  a Kicker 06 S15L7 D2, and i'm reading the manual to get the specs for optimal boxes and what-not. Well, not only does Kicker list the box volume, but the maximum watts (rms of course) in the same table, which i thought was a little strange. I look at the page for sealed box specs and then the  vented box specs and the max power ratings are different for the same woofer. So basically what i'm wondering is, is why is it that the max rms for a vented box is 750w, but for a sealed 1000? I'm not completely ignorant when it comes to car audio install, i've done all my cars and helped countless friends with theirs, but i've never seen different power ratings based on box type. I've installed kicker speakers before, though not subwoofers, and i don't remember ever seeing different power ratings. Are kicker subs the only one? And why would box type affect the sub's power handling capabilities?

A sealed enclosure will provide the sub with resistance as the cone is thrown forward and backward where as a vented enclosure won't hense the power handling difference.  If you push too much power through the sub in a vented box it'll tend to bottom out damaging the voice coil former.