Bethesda made design decisions during production that make me believe that they are attempting to target the action adventure gamers in order to broaden the base. These gamers tend to buy the game (important part), rush to complete it, and then put the game down.
My point is that you can complete the main quest without getting a skill to 100, getting to level 50, getting married, or finishing the guild quest. Therefore, these numbers do mean something.
You can only get married by doing an obscure quest and unless you hear about marriage on the internet, or in the press, getting married is not something that jumps out. The fact that 22% have gotten married is significant, and is also something that the action adventure players are generally unlikely to attempt.
The guild quest is much less obscure than getting married. You can get on this quest quite accidentally and you do not have to go looking for it. Yet, only 7% have completed it. The people who got married were typically not interested in the guild.
I think that the 28% completion of the game represents two types of gamers. The first type is Bethesda's new market, the action adventure gamers who go in, finish the game, and are done. These people are probably not represented by the other numbers. The second type is RPG gamers that rush though the game, probably to experience it without modification one time.
The inverse of the 28% (the 72%) includes too many types.