It depends where u live basically. If ur in a city.. then yes, most likely it is due to smog n all that... out in the countryside is a lil better... most rainwater is 6-7ph in 65% of us/canada I saw it on the discovery channel! haha
what about a/c condensation water? i know of a car dealership here in hawaii...that collects all their a/c condensation water in barrels to wash and clean all their new cars. they claim its a cleaner water / water spot less free water as well as saving them a utility bill.
they also use a diesel to run the dealership which they run and make biodiesel getting used cooking oil from their neighbor Wendy's
im just thinking about collecting a/c condensation mixed with a little of my uncles koi pond water. its amazing to see the growth of the plants where he dumps his koi pond water when hes cleaning.