Causes of different AFR

I have a flat 6 cylinder engine and for some reason one bank is running 15.5 and the other is at 13.8. I had the engine in a engine dyno and convinced myself that it was a problem with the O2's on the dyno but when the engine went in the car I was sure it would be right but it wasn't.

My question,

How does spark and fuel effected the AFR. In other word

If we get the correct amount of fuel to air but it isn't all burn't what will the AFR do? How does does this effected the AFR,

Any smart ideas on how to find the problem?

Typically if you've got a problem like this it's related to air/fuel distribution to the different banks of cylinders. A very real issue that I've seen a few times now on V configuration engines (or flat 4/6) is if the cam timing is different on one bank to the other then the engine VE will be different and this will result in a different AFR bank to bank - Actually what you'll see is one bank start leaner than the other and then as the rpm increases it will begin to move richer.

Other issues to look at would be your injectors - Are they all flowing evenly? Is there a mechanical issue with the engine? Are the compressions equal for example? If you've got a weak spark you're much more likely to simply end up with a misfire that will be audible and this will read lean as oxygen will be passed into the exhaust system.