I think it was a similar situation as the Simpsons being neighbors to the Flanders. As I recall from that show, Ned Flanders actually earned less annual income than Homer Simpson, but had a nicer home and more wealth, because he spent it more prudently.

The implicit assumption in MWC is that no matter how much Al Bundy might earn, Peg and the kids would always spend 101% of it, leaving him with -1% to spend on himself. That makes Al, specifically, the poor guy in a lower-middle household; working all the time with nothing to show for it personally.