Gaucho:
I think they are trying to keep the highest number for their flagship. It'd look silly if in two years they develop a 50 seater and they have to call it A360 when the big guy is called A350. This will be the flagship perhaps for as long as 40 years (look at how long the 747 has maintained its leadership position).

But the 747, since it was developed, has been followed by many smaller aircraft with " larger numbers": 757, 767 and 777. Don't make sense to me.

Airbus is smarter: they are giving themselves some leeway in developing up to 3 new classes of aircraft (350-360-370) and still have the 380 remain the biggest guy both in name and number of passengers.

Just a guess.