What Orion said: the advancement skill is on top of whatever your normal skill level is. I probably didn't phrase that as well as I should have
In time, I'm going to increase the limit to +10 for primary skills, +8 for secondary skill and +6 for everything else. I just limited it to +5 for the time being to get a good idea of if the advancement rate is good or not, and I think its about right (side note: currently, the highest advancement level is one person with +4.4.)
Also, as I previously mentioned, magery skills (magery, invocation, necromancy and animal lore) will be getting advancement also, but before that, I'm afraid I'm going to have to announce an upcoming change that I'm sure will be unpopular: From now on, casting 7th and 8th circle spells will require magery as a primary, and 5th and 6th circle spells will require it as a secondary.
Finally, addressing tradeskills, although I'd like to put in this sort of thing for them, I'm really not sure if it would be possible to balance them with an additional +10 skill. +10 skill in a crafting skill is a HUGE deal that makes a major difference in your success rate, far more so that +10 skill does in combat skills (this is actually the reason with the above magery thing too. Giving everyone even +6 magery, in addition to the +10 from items, makes everyone a mage, more-or-less.) So, I don't know yet...