I think it has something to do with the date the return is first transferred/accessed. 

My own return is one of these ones that are split up in HB now, I transferred and accessed the federal return in really early January but the state return I didnt access until the end of January...so Im my only problem child right now  LOL

But I see other clients with 2 different dates of transfer that still sit right next to each other in HB, so Im not sure I understand it completely.