Small wheel issue

My Magene sensors for my 20" cumbie that I have set up as a stationary trainer , give speed and distance wildly different from my PowerTab hub (Pro+)
This was brought out when I used the https://www.bikecalc.com/speed_at_cadence
It correlates well with the Powertap, but is 20% off with Magene using a Karoo K1 head unit.
The tire is Kenda 20x1.5 that is C/W a Kwest but not model labeled.
Measured circumference with calibrated tape was 154 cm. (1540mm)
Charts all seem to have been copied from an unknown source give 1490, except Cateye which uses 1513.
I have Lots of variables still to work out, but I was wondering if small wheel folks (cumbies and foldies) were having an issue with speed and distance accuracy?
As tire/wheel circumference may be off by a enough to make a difference.
Doubt if foldy folks would go down this rabbit hole,
but cumbie trainers are more likely to have seen this, if it is a real thing.