If you ever used a console-command that counts as "Cheating", and disables all future achievement logging.
The cheat is recorded in the save-game, so it will follow you forever. Even if it is just a game-error, and you didn't actually cheat. (Not that you cheated, you may have used the console to fix your game, unsticking yourself from being stuck, or removing a quest item, or forcing a quest to complete which was not working due to a bug. Or, just simple game-save corruption, which is 100% common in this game. Though you may never actually see any corrupt issues in the game. Depends where the file got corrupted.)
Not sure this is necessarily true as stated as I used to console command to fix an issue I was having with one of my companion's weight issue bug - http://forums.steampowered.com/forums/showpost.php?p=26546424&postcount=10 Anyway since doing that, I was still able to get an achievement for finishing a quest for the Thieves Guild.
With regards to not getting an achievement while playing offline, you might have a point. However Steam does try and synchronize with the game when it comes back online, but it might not gather all files. Not sure here as I've never played offline.
Anyway it seems that my issue - http://www.sevenforums.com/gaming/198691-discuss-skyrim-34.html#post1695168 may be due two the fact that some small requirements still have not been met or completed. Even though I've completed the main quest and the civil war I'm just discovering certain conversations about the execution in Helgen from NPC's in Solitude. Could the be why "Unbound" still shows locked???? Don't know.
As I said earlier I still have Unbound, Bleak Falls Barrow, The Way of the Voice, and Gatekeeper (join the mage guild) still showing locked despite being done. Now I'm begginning to wonder if there aren't some small tasks that need doing with these that aren't evident?