The head count question is easily answered. You are counting passengers, not seats. Ok, by the powers of deduction most of us could easily ascertain the passenger count but at the end of the day, can you say that you have counted the people? No. So what happens if you are counting the empty seats.... 1, 2, 3, 4 and then you get to the back to find someone in the toilet and someone waiting to go in? Quite literally it adds another equation to the process and this could ultimately lead to a wrong headcount.
Also, as someone else has mentioned some crews operate on 1 type of a/c with multiple config or operate on 3 different types before you even go into sub types. (i,e 757-200/757-300). Whilst the crew member should know what a/c they on................and they will do, a momentary lapse or confusion just makes things more complex.
So as you can see, simply counting seats is not quite as easy as some clever cloggs make out.
See bizdev? I told you so.