I would hazard a guess also that many who identify with a religion don`t actually practice the religion they identify with .
So wouldn`t / shouldn`t these ones really come under the scope of being of no religion ?
I`m sure we all know many people if asked what religion they are would give you an answer to this or that religion but have never gone to church and have no intention of ever going to one.
That could be ascertained by asking the questions ,
how often do you attend ,
when was the last time you attended
and are you planning to attend in the next 5 years or so .