I think I finally understand how skills work. When you start the game, you have a number of skills that you are trained in. Skills that you have trained will increase your base chance multiplied by 2 (Base chance level x 2) and skills that you are untrained in will only have a base chance based on the attribute (no multiplication by 2). However, during game, if you rolled a 1 or 20 (Dragon or Demon) for any skill roll, you can mark that skill. During time of advancement, you can roll a d20 and if you rolled more than your base chance then you get to add a skill level for that skill which increases the base chance by 1. If you roll lower then no skill level is awarded. This reminds me of BRP skill advancement (Call of Cthulhu, Runequest, etc.). It seems to match because both Dragonbane and BRP have a shared genesis. A skill level can never be higher than 18.
Man, the rules are scattered here and there in the book.