I think what the OP means is why does the upgrade list show 15 available coach seats on a certain flight, while ExpertFlyer shows only Y1.
When we are added to the Upgrade List and click view upgrade list we have pop up that shows open seats in F and open seats in Coach. I think that is what the OP is referring to- not the number of Medallions on the upgrade list for F.
Personally, I'd like to myself. Many times I see ExpertFlyer showing close to no availability and I'm excited for a possible oversold situation, yet Delta.com's upgrade list pop-up shows many coach seats available. Why is that?
-Kinglobjaw