Why a false PH reading?

With the help of 6" of rain and some HTH PH down, my disaster has been corrected but I'm curious to what led me to such a screwup. When we opened the pool (17x36 oval) it looked very good and after vacuuming the small amount of sediment and backwashing, I was ready for the chemicals. I was expecting a low PH and a zero Chlorine but what I got was zero (clear water readings) on both. I added PH up until I knew something was wrong. I added the packet of shock and only then did the PH show a reading and it was a whopper. Everything is OK now but I am curious to why there was no PH reading until there was chlorine available?