The 7 miles per dollar comes from the delta Amex marketing flyer sitting on my pile of mail, but I didn't look at the fine print, I got curious and looked into details : actually it's only 1 mile per dollar for usual purchases, you get one additional mile on "delta purchases", then you can get 5 extra more on money spent on flights itself. So yeah on average it's close to 1 mile per dollar, I can't edit the original post. So the math makes more sense with those numbers :)
