I'll put money on the fact that the calculation is *not* incorrect.
What is the prize for whoever guesses what the content of the various cells
is, as you haven't told us?
The starting point is to split your formula into manageable chunks.
Does =COUNTA(AL10:AL8002) give you the value you expect?
If not, have you considered whether some of the cells may contain a text
string which might only contain spaces?
If in doubt, try a helper column with =LEN(AL10) and copy down and see how
many non-zero values you have.