Different carbonates have different EC values. The biggest cuprits in hard water are Calcium and Magnesium.
Lots of cities use Limestone clean the water, so your tap water will have a bunch of Calcium in it.
Calciums EC value will be different from say, the EC values of Nitrates with in the store bought premix.
The bottled premix uses a very specific combination of salts to get the values of EC that they're specified for, so if you mix them with Tap Water, your throwing the EC values off substantially and the total EC or ppm or CF or whatever, is accumulatively inaccurate.
This inaccuracy will increase even more with the combination of salt sources, especially if you mix name brands.