Joe, I know that in my area, people with swimming pools and no retractable cover have lots of problems with phosphates in the water, presumably caused by rainwater.  Those with retractable covers that pump off the rainwater do not have as severe a problem.

Perhaps some rainwater in your area has picked up minerals from the atmosphere that is good for your flowers?

We have a coal fired power plant east of us, even with scrubbers that are supposed to remove particulates from the stacks, they are said to add sulphur to the rain west of the plants. (due to prevailing winds)

I do know that the rainwater of today is not the rainwater of my childhood.