Teflon seal rings : Huh !

I've always used iron seal rings in my trans builds , because the teflon ones didn't look right to me .
But since the trans was in pieces , I did an experiment . With the front clutch mounted on the pump i did an air test of the clutch pack , first with the iron rings , then the teflon .
Result : the clutch pack clamped fine with the iron rings but not at all with the teflon rings .