With a normal fuel load and no malfunction a scenario where you might get a LOW FUEL with an imbalance is leaving the crossfeed valves open.
Over time one side inevitably has more boost pump pressure than the other and an imbalance occurs much faster than with the crossfeeds closed (or so I'm told
). However, you should get a FUEL CONFIG when the imbalance reaches 2200 pounds, normally well before you get a LOW FUEL when the useable fuel drops below 2200 pounds in either main tank.