I think the PT session asks for business casual because most evening students will be coming from work wearing business casual.
Maybe the full-time session expects that students will want to go out later on and possibly stay the night... maybe they assume more people will be coming direct from college and so they've lowered the level of formality.
I was at a reception for newly admitted students last week in DC and everyone showed up in suits, most of whom were PT students.
I know this doesn't really answer the question but it at least fuels the fire a bit...
I'll be in DC on the 24th, hope to see everyone else there too.