Greg: Read this to see if maybe the lab didn’t do the best test for finding AeT.
For someone with a good aerobic training base like an ultra runner I’d be pretty comfortable using RER=.85 for the AeT.
However I would also perform a field test to determine your AnT/LT. I MUCH prefer this as a way to determine this upper threshold.
It is certainly possible that your AeT to AnT/LT spread is 3%. Especially if you have done a ton of Z1-2 and little high intensity. If it is you should for sure start adding more intensity sessions.