I'mma tell y'all this one more time:
Don't discount the notion that major labels dont even ship out that many units with certain artists anymore to begin with.
I know that sounds like a cliche excuse, but ask yourself if you even see that many copies at your local store...shyt, do you even have a local store anymore?!
I think distribution downsizing is just as much a factor as downloading. I'm not saying Big Boi was gonna push that many units anyway cause it wasnt even heavily promoted or had buzz like that (Where did you even hear the Kelly Rowland single?) but I think it plays a big part in why some artist have low numbers THAT LOW compared to others.