IMSA Overall standings - confusion

Hi everyone!

I need some clarification regarding the IMSA WeatherTech SportsCar Championship series, particularly concerning the overall standings.

I've recently begun following the series, but I find the standings rules a bit unclear to me.

After each race, points are awarded for each class and drivers, whether it's GTP, LMP2, GTD PRO or GTD. However, I'm wondering why not all drivers are included in the overall standings which is shared on an official website.

Thanks in advance!