There are two ways a team can score at a high clip despite averaging relatively few yards per play. The first is to start possessions close to the other team's end zone. That is not the case with Marshall. The second is to be consistent on first and second down to get short 3rd downs (e.g., completing 70% of pass attempts), and converting on those 3rd downs. In fact, without adjusting for strength of schedule, no team in the country was better at converting on 3rd down than Marshall. This allowed them to extend drives; 6.5 plays per possession was the second most nationally. Combined with a super fast tempo, Marshall was able to score 41 points per game.
But they allowed 43. The Marshall defense was bad in almost every way, but also symmetrically so with the offense: high completion percentage, many plays per possession, etc.
The Statistical Review breaks down teams along a number of performance categories, everything from red zone scoring to field goal percentage, and compares that performance against the rest of the FBS. All 124 teams will be reviewed from 124 to 1 by the hybrid rankings. You can find short descriptions of the stats used in the table below.