I'm not a mechanic so I can't explain why it would read for only one bank. I do know that my 4.4i had a fuel trim too lean bank one code and replacing the MAF solved the problem. As the part was so expensive, $350, I did a lot of research and the MAF was at the top of the list of causes (behind the fuel cap).

As for causing it to it being a lean/rich condition that would depend on how it's failing. Readings from the MAF are used to richen or lean out the fuel mixture depending on the air density (or what it thinks is the air density). A MAF works by heating a thin element to a high temperature. As air passes over the element it tends to cool it down. The amount of current used to maintain the reference temperature determines how dense the air is. Denser air cools the element more thus requiring more current. Less dense air requires less current. Thus the fuel mixture can either be leaned out or made richer.