Haltech WB1 and AEM UEGO reading slightly different AFRs

I have two Bosch LSU 4.9 AFR sensors mounted in line about 2-3 inches apart in my exhaust. One is connected to my haltech elite 1500. The other to my dash mounted AEM UEGO gauge. Most times they read bascially the same. Sometimes at idle, they read 0.2-0.3 AFR different from each other. Sometimes while driving, they will read as much as 1 AFR different.

What is the cause for this? Can it be fixed? Which reading should i trust? The haltech AFR sensor is mounted upstream.

sensor grounds, calibrations correct? exhaust leak would be my 1st checks, they always jump around slightly perhaps 1 is lazy :)