It may be that added pressure at a localized point on the fabric wall expands the fabric and coating a little, while increasing the water pressure at that point, or both.
It points up a big problem with the Suter-type testers, unless they have a control for this. Roger tested some EPIC for me to around 1500 mm with a Suter-type tester; but we both know that with applied pressure, as with seam stitching and/or tent poles, it is nothing like that.
Sharp creases also have a marked effect, although I suppose you can set up a tent without sharp creases on the canopy.
Maybe we need something like a pencil eraser sticking up against the material in the Suter-type testers. Guess with the very high HH on the Thru-Hiker silnyon, not to worry. Wish Paul could get a light green or tan color that is more stealthy. The brown and dark gray will make great ultra-light tent floors, though.
Returning to the OP, the problem is that most silnylon sold today has quite low HH.
Many threads attribute this to envionmental regs, and a recent one directly to use of chemicals that affect the ozone layer. So does this mean we are being anti-green by using the stuff sold at T-H. Another conundrum.