This has been an issue for us for a very long time. We've had several "web tech" people look at it with no solution found. They've looked at our code and haven't found the problem.
First we noticed how varied the data was between our analytics and adwords. Then we found in analytics that it gives odd quantities to products within a transaction. For order #12345 it lists the products sold and sometimes the quantity is right and sometimes it's way off. When I look up order #12345 in our admin the products are correct but the quantities don't match which makes our numbers way off.