odd stack temp difference/

cleaned my boiler last nite, long overdue, stack temps were up, too much fly ash in the tubes and on the turbs. So i added another magnetic therm on the flue pipe. one about a foot from the back of the boiler on an elbo and one about 2 ft from that one on the corrugated stainless liner that come out of the masonry chimny. The second temp is 100 or more degrees higher. i switched therms and got the same result. 100 or more higher 2 ft further away.
heavy gauge pipe 8 inches off the boiler then 1 elbo (right hand turn) (first therm) then a transition into stainless T. Cleanout cap on bottom of tee and chimney liner( left turn) on other side of tee and 2 ft up into the chimny. is the temp diff because of different metals, stainless vrs steel or is it a matter of flue gas stalling at the tee and giving a hotter temp. normal stack temp when just cleaned at full burn ranges from 3 to 450 depending on wood quality. i have too much draft so i have all 8 turbs installed when i have good dry wood like this yr. with less dry wood i remove a few turbs accordingly. For an old tarm it is pretty efficient when using all of the turbs.

I've found that the magnetic therm's (which I do use) are impacted pretty heavily by drafts and close by exterior walls. Is your first thermometer getting a draft? Is it closer to a basement wall than the other one? Type of metal can certainly also impact it...