productTotalExVAT function returns sum of all items and give total. But if you have lot of small items, you have a difference with real total (as you can have basket class).

ProductItems function stores round values (L836-837 and 845-846) instead of exact values and sum these round values to return total and you have approximate result.

I have problems with many (20) small items (<5€) . Remove these four lines solve the problem.
It's a big problem for invoices ...