I'm speculating, but the pattern seems to be.
PC players who bought horizons get the 2.x version of the code.
PC players who didn't buy horizons + Mac + XBOX are on the 1.x version of the code
Horizons 2.0 and Base Game 1.5 are the current releases
Horizons 2.1 and Base Game 1.6 is soon(TM)
Some new features such as Engineers, looting and crafting will be exclusive to 2.1
Some new features such as the improved missions will be available to both 2.1 and 1.6
Bug fixes will apply to both versions 2.1 and 1.6
At some point in the future, horizons will be available to XBOX CMDRs
I imagine there are a couple of issues holding up the release of Horizons on XBOX
1. Performance. the planetary generation takes it's toll on the graphics card. I own both the PC and XBOX versions. My GTX 770 2GB struggles with the PC version of horizons unless I turn the planetary detail down a lot. From what I can fathom, it's the amount of VRAM that makes the difference, rather than the speed of the GPU. PC CMDR's with GTX 770 4GB seem to have much better framerates. The graphics card in the XBOX is above the minimum spec of the PC version, but below the recommended spec, so I imagine they are trying to optimize the graphics so that people don't complain. The game looks pretty awesome on XBOX and I imagine the devs want that to continue.
2. The sales model. On Steam Horizons is sold as a separate game. You get a discount if you own the base game but Elite: Dangerous and Elite Dangerous: Horizons appear as 2 different games in your steam library. Does the XBOX store support this? I.e. can you get a automatic discount on one game if you own another? Or will they need to rethink the model and have Horizons as an upgrade.
Think positively, by the time Horizons comes to XBOX it'll contain a lot more than 2.0 did so it'll feel like a much bigger update.