I've already received what I have been referring to as the "screen warning" e-mail (the likes of IU, UW, Carverwassup w/ the Midwest schools hating so?) that explains UW's cut-offs (2.8 GPA & raw MCAT#'s 7,7,7).
Given your 6 on a section--even though that's lower than many--they couldve sent you the secondary for some other positive reason.
Are you sure you got the official invite? If yes, I--being a person that has not seen that invite--would go for completing it (and I probably wouldnt even ask them why they sent it beforehand, if you have the courage and time to just git er done and send it offmaybe when youre done w/ other apps), b/c who knows what screening reeeeally means, afterall. It may mean many things.
Perhaps you rizocked the PS, and they want a piece of you badly, warming an interview seat. Just my thoughts. I honestly dont know how strict screens school are with regard to their cut-offs as they advertise them, but this may be a
hmmm
.sign?