I think you mark xp whenever you roll the right number, and then gain a level once you've gained enough xp. So when you're level 1 and roll a 1, you get an XP and then level up to level 2 (since your xp == level). At level 2, you gain an xp the first time you roll a 2, and then gain an xp and level up the second time you roll a 2.