Airflo Compact Scandi vs. Rio Scandi Spey Head

I am trying to find the perfect compliment of lines for my Burkie 7127. I purchased a new Rio Scandi Spey head 420 (31'). Fished with it a week. Casts and fishes fine, not great, but fine. I broke the tip off the rod and sent it in for repair. I just got it back (very quickly!) and I've been going back to the drawing board, trying to get the line dialed in.

Since it was in the shop I went back to my Beulah 6126 and had fallen in love with the AFS heads. I tried the AFS on the Burkie when it returned and it cast well, not great, but well. I dug through my bag and pulled out a Airflo Scandi Compact 420 (32') that I've had for 5 years. I lined it and it casts great. I get a nice wedge -- BOOM. Great line. Yes this is the line that Burkheimer suggests for the rod, I know. Sometimes I just don't listen that well and I have to go through the ringer to get back home.

I honestly didn't think that the difference in tapers and grain displacement would be that different between the Rio line and the Airflo line. They are a foot difference in length, but something tells me that there's more to it than that.

Anyone put a micrometer on these heads and find out what's really going on?

thanks!