I think many of the nones simply don't think about it most of the time and only when questioned do they have to think about how they identify. Secularism has been on the rise for decades and with the rollback of religious themed activities in the schools and in popular culture it just isn't on most people's screen on a daily basis. Most are thinking about work, kids, bills, vacations, etc and religion or spirituality isn't even an afterthought.
I was talking to a young boy a couple of houses down and he had no idea what Easter was. I asked if they did anything in school for easter and he said no. His parents don't go to any church but have said they are believers. Religion, at least here in the states, has just largely fallen out of the general consciousness of the populace, especially those under say 30.