ABC classification when using project module

We use the project module, and invoice our customers directly from this module, i.e. we do not use sales orders. It seems to me that the function calculating ABC codes (as well as inventory turnover rates) does not regard items sold via the project module - at least all our items are in group C when I calculate.

Is this correct?

Does anyone have experience with calculating inventory turnover rates or ABC codes when using project module?

Thanks in advance.