EFC is often $0 because as a graduate/professional student your parents' income is not taken into account (at least it wasn't when I applied). You will get less priority for institutional scholarships/grants than people from low-income/disadvantaged backgrounds, but you can still be eligible.
The main reason for the discrepancy is what the above poster stated--lots of students who didn't have to borrow money in the first place. Lots of my classmates had two physician parents, drove Mercedes, etc. Many had parents wealthy for other reasons. They didn't have to borrow anything.
I had an older beat up pickup and thought I was doing pretty well with that... Either way, I borrowed the full cost. I got jealous of my wealthier classmates at times, but after I thought about it more I was actually quite happy for them that they were fortunate enough to not have to borrow the full cost of med school.