I have told so money people abput them test till i finally did one my self and on man it got deep quick 
But just tell people to say they rent even if they own and they want try to push in on them. Tjey only want home owners, but they would still get their test of course…
So the water company test is done at there site.
The water then goes through sometimes miles of pipe and possibly storage tanks before coming out of our sinks right, so i would imagine there could be some differences but the online stuff is a good place to start…
I would have to say that you are correct on that. So I was having issues with my first grow which I’m still in the middle of and when my TDS meter and pH pen came naturally I tested my tap water and the pH was over 8 and the PPM was like 410. So I did some investigating and that’s where I found this water report. But I would 100% have to agree with you miles of pipe you’re in in house piping there has to be some deposits that will come with all that.