A lot of these tournament has variable blind structures so levels become 12 mins and then 15 mins maybe even 20 mins if its a big field. As there are smaller increments to the levels, you may be playing deeper than before when you reach level 15 or so.
Having said that, a few of my favourite tournament were the 15 mins blinds regular structure, there was a noticeable difference, and they seemed to play deeper in my opinion so sad to see them go