If you had done that by adding baking soda, it would have taken more baking soda than you dosed of macros to get that much KH rise - I forget the actual amount needed, but as I recall it would be around 3 tsp for that much water. KH2PO4 and KNO3, if that is all you dosed, wouldn't raise the KH at all. So, I suspect your substrate or any rocks in the tank are the culprit.
EDIT: It occurs to me that, while carbonates are what our KH is intended to measure, it isn't only carbonates that will raise the KH, which is actually the alkalinity that is being measured. I think phosphates will raise it too. If you made a mistake and dosed KH2PO4 thinking it was KNO3 you might have raised the KH some. (I could be wrong about this.)