What causes the variations in Earth’s precipitation during the course of a year or from year to year at a given location?

As the Earth revolves around the Sun during the year, the orientation of its axis relative to the Sun changes. This causes the apparent position of the Sun relative to the Earth to change, and creates distinct seasons.

Explanation:

I believe the answer to this question is the filtrate is reabsorbed.
This is brought into the system because it still contains the useful products for other biological processes.   Thank you for your question. Please don't hesitate to ask in your queries.