Consider this...
In the summer students are free of other obligations (particularly full courseload) and consequently have more time to study for the DAT. If you took the DAT during the school year, people would generally be less prepared becuase they wouldn't have studied as much. Thus, if you know the same amount of material in both a November writing session and a June writing session, you would be in a higher percentile in the November writing session because everyone else is comparatively worse.
It makes sense however.. this would really only work for someone if the person could take a year off school or something so they dont have the obligations everyone else is faced with.