You forgot to account for the duration of Ascendance -- the CD starts immediately on activating it. This means at the end of Ascendance, your CD should be showing 2:30. Your additional lava bursts will reduce that furher, putting the effective CD at between 2:10 and 2:30.
Game it out. To get 2:30, I assumed a cast time of ~1.2 sec in Ascendance, and an interval of just under 7 sec, which turns out to be an overestimate of the interval (my bad). An interval of 5 sec (what my most recent simcraft results showed) results in a CD at 2:20 +/- a few seconds.
The overly simplistic (but still reasonably accurate) rule is that the CD is 2:00 + 0:05 * ([interval between lava bursts in seconds] - 1).