Then explain to me why these denominations are analyzed as "single religions" for recording purposes.
You answered your own question - for recording purposes only.
Why refer to it as a religion then?
I consider my Catholic church to be a religion with it's own beliefs, leadership, etc. within the larger faith of Christianity. Just because I consider myself Catholic doesn't mean I don't consider myself part of the larger Christian faith.
So you are differentiating between two types of religion, which we can call the "greater religion" (Christianity), and the "lesser religion" (Catholicism). That makes sense, I guess. But shouldn't Islam be compared to the greater religion, rather than the lesser religion?