Household cleaning products are 80% chemical water conditioners. Calcium magnesium and Lyme are the sworn enemy of wash stuff appliances. Rain water has a decimal value of hardness and extremely low TDS. This makes it a really effective solvent when it comes in contact with the cements etc. Those things splash onto your bicycle. A really good example is black fender wells. That haze on them is the dissolved solids from the road. Washing the bike first is important. I typically blast out the frame tubing with my pressure washer. I also check the water again after the bike has been in there for a little bit. The scaling will dissolve quickly into the bath and if there is enough of it it is going to raise the acid demand. It's not really critical if you start with a 2pH target but if you are just using litmus paper or test strips you can't really get that accurate a measurement