First off, Paradox has chosen not to discuss MP issues in this sub-forum. It will undoubtably be moved to the MP forum to be responded to by other players. I am one of those other players, and as per their thoughts with this policy, I've been where you're coming from.
The reason that your "v1.3b" game appears as "1.3a" is that the 1.3b patch just doesn't change the '1.3a' in the main screen. Thats because it isn't a "major" patch.
It does, however, change the checksum. The checksum is a means of insuring that all players in a multiplayer setting have exactly the same basic files defining things such as how much an infantry division costs. This is to prevent cheating. These files change between versions, so you can't play with anything but exactly the same files. Eg. 1.3a and 1.3b can not play MP together.
Your 1.3b game attempted to connect to a 1.3a game. You had different checksums, even though both of your games claimed to be of the same version (1.3a). So it didn't work. Get the host to upgrade to 1.3b if you try to play again.
If you have a question like this again, post it in the multiplayer forum. If you don't find an answer quickly, consider sending a bugreport through the procedure described in the support forum. Ie send a save and description to
hoi2bugs@paradoxplaza.com
