His answer, as told to me by borgmama1of5 is as follows:
They decided to do "soap opera time", because they didn't want to have to push the year forward for the rest of the show's run. So, just like in soap operas when people's kids are toddlers and then suddenly teenagers, so too can Sam and Dean spend a year apart while no time has actually passed in reality.
So, it sounds like it was indeed a conscious decision, though, he could just be good at covering their tracks. On the location tour today, Russ Hamilton was saying how the crew are always really impressed by our attention to detail. He said he was talking to Guy Bee about our indepth questions. (On Sunday, ratherastory asked Guy Bee the question about the timeline for me during his panel, he did not have an answer). Russ joked that people will get fired for their lack of knowledge, saying "How we didn't know that a year after 2010 is 2011, I don't know."
Either way, what's done is done. I'lll continue to push all my fics forward a year, but I'll keep the timeline true to the show.