In ProMix, for example, the peat is the part that is most acidic. Now, if your tap water reads over 7.0 but has very low ionic strength and EC, that means there's not really a lot of stuff in it to react, and it will be influenced MORE by that peat, just like RO water is, than say a tap water that reads over 7.0 and has a lot of carbonate ion in it.