There isn't any pattern I can discern and it actually bugs me quite a bit. I've tried to find previous threads on this topic a couple of times before. A few schedule "changes" I've had, the only thing different was the flight number. Same departure and arrival times, same aircraft, and it didn't become a through flight.

Looking at several routes out of RDU, there is some consistency in places for odd/even, RDU-DFW, but not enough to set a precedent IMHO. There are several routes with even and odd flight numbers.

RDU-AUS: 4688, 4647
RDU-ORD: 1749, 435, 1919, 1634, 597
RDU-DCA: 4629, 4654, 4670, 4669, 4631, 4630, 4633, 4700, 4678

Does time of day play any factors? Maybe flight numbers tie somehow into crew scheduling? I hope someone has the answer, assuming there is one.