That's a legitimate question! If most of the roommates were gone, shouldn't there be less usage when lights were off and appliances were not in use? While this is a correct assumption, a majority of utility consumption will occur regardless of the number of people still living in the unit.
For example, even if just one person remains in the unit, the amount of energy it takes the A/C unit to cool the entire apartment remains the same. Likewise, water heaters use the same amount of energy to keep your water hot 24 hours a day, even if the water is not used.