This is a great question. As you probably already know, greywater is waste water from the home that is not toilet water. So for example, your water from dishes, showers, sinks, and laundry is all greywater. From what Ive read, it looks like greywater has some nutrients in it that can actually help plants grow, which explains your observations. The water you drink goes through many expensive processes to remove contaminants and minerals to ensure that what youre drinking is just pure water. However, plants dont need their water to be that pure, which is why greywater irrigation systems are becoming more and more popular. In these systems, the greywater may be filtered or treated to remove large contaminants, but it is still much simpler than the whole purification process used for drinking water. It saves quite a bit of money to be able to use water that hasnt gone through those expensive processes. And, as youre seeing, in some cases the nutrients and minerals in the water might even help plants grow better! If you want to learn more, you should talk to local farmers at the farmers market or a local farm and see if any of them use greywater in their growing, as well.