There is a guide book already? I never really need it, I just really like to see what it says about certain things. Before I had internet I had to use the Morrowind guide book all the time.
Yeah, I got the guide book at my local game store since it was heavily discounted because I pre-ordered the game. Its 655 pages.

The guide book says:
Skills: All Followers have a specific set of favored skills, which improve as their level increases. Unlike your character, Followers do not become better at the skills they use - no matter how you try to force Morcurio (a mage) to be a greatsword-wielding warrior, he'll always be better with magic. Make sure to select a follower hwose skill set meets your needs.
Levels: Most Followers automatically level up when you do, so there's no need to worry about a Follower 'falling behind' if you go off and adventure on your own, or want to work with someone else for a while.
Hope I don't get in trouble for quoting from the guide book....
The two parts in bold almost seem contradictory. But I think the second bold just means that unlike how it is for me, it isn't the
use of a skill that improves it for the follower, but rather it is their favored set of skills that improves independent of which skills they use the most.
I wonder where the info posted on UESP.NET is coming from. If it is testing, then the guide book must be wrong.