Different question: Why shouldn't it be the case, i.e. have the same timespan per incarnation?
Don't forget that all these three regenerations weren't triggered due to any natural/inherent reason (like dieing of old age). The ninth Doctor absorbed the vortex energy from Rose and the tenth Doctor absorbed the radiation from that completely weird and odd failsafe containment thingy. Neither of them regenerated by choice.
Also keep in mind that timespans are hardly constant when comparing single episodes. While this is especially true for time travelling stuff, it's also true for pretty much any other TV series or movie (maybe except 24).
For example, take episodes such as Rose or Aliens of London. The story happening in those essentially covers a day or two each. Compare this to episodes with far longer timespans, e.g. The Impossible Astronaut and the sequel: Those cover several months. Or, if you want it even more time bending: Blink happens on a single day for the audience as well as for the characters depicted in the present. However, for the Doctor (and Martha) obviously months pass.