AFR readings

It's the heat.
It's so small, the heat is less dense air...so it would/could be rich that day while spot on the previous.
It could be leaner, yes, for economy.
For et? You'll have to test that out now.
Change 2 sizes smaller on the primary if you want to venture down that path and find out...
Or jet down till its lean surging at cruises then go up 2 sizes and test at the track.
Might find some power, may lose some.