I've seen a lot of argueing by people about the unannounced PC release date and many are confused about why the release date is not shared with the XBOX. Their main argument is that Microsoft only had an agreement for having 2 DLC's (DLC = expansion) released on their XBOX a month ahead of other platforms, and with Dawnguard and Hearthfire out there that agreement has been met. In truth however, only 1 DLC has thusfar been released, despite of what people may think. Here is the technical why-and-how:
The game files are what holds the content of the expansions. When you pay for and download an expansion, a file is placed in your Skyrim directory which adds new quests, areas, items and so on. Inside these files is a lot of code, which makes all this new data work with your game and that very code is tagged with a prefix to identify what kind of data has been added. Dawnguard has the prefix "DLC1_" in front of every bit of data that is added through the expansion. This defines that the added content is a DLC. The 1.8 patch adds some data that can be used when the Dragonborn DLC is released. This data has the prefix "DLC2_", which defines it as the second DLC. So what about Hearthfire? Hearthfire has the prefix "BYOH_" in front of every bit of data it adds, which is short for Build Your Own Home. Since Dragonborn is prefixed as DLC2 and not as DLC3, this defines that Hearthfire is not a DLC at all. It is a content pack, no different than a pack that would add say, horse armor.
So why did they release Hearthfire and Dawnguard as two separate items? Likely because of a promised deadline for release to Microsoft. The content of both Dawnguard and Hearthfire were showcased at the Bethesda Game Jam and most likely were meant to be one big DLC together. When the release date came closer and Bethesda had those issues with the PC adaption and PS3, they probably decided early on to release the build your own house content as a separate pack, Hearthfire.
I hope that clears up why the Dragonborn DLC for PC is almost certainly released a month after the release on the XBOX.