One trouble is, very often, every aeroplane in the fleet has a different configuration. On the 744 we have big and little club configurations. Then some seats are u/s and blocked out etc etc. On a 737 or A320 it is a much easier task. On a big aircraft it is a fraught with errors.
Sometimes 2 people are in one seat... Mother and baby. People move whilst the count is taking place, despite being asked not to.
To try and make sense of the current security regulations in aviation is going to drive you mad. It is mostly designed to keep the Daily Mail editor happy as far as I can see. We, as in we the crew, pilots and Cabin crew, have to endure airport security multiple times a day. It is as bad for us as it is bad for you.
I deliberately bid to avoid the USA because of it.