Point in favor of this interpretation--T3 levels were intentionally shortened (XP-wise) to make them below-trend and faster. The word of devs intent is that T1 goes by quite fast, T2 lingers, T3 is again fast, then T4 is the big hurrah (if you get there). Effectively, you have two segments:
* The "main game" : 1-10.
* The "secondary game" : 11-20
And 1-4 and 11-16 have the same feel--introducing you to the style there but not being the meat.