PoolMath has worked well for me for all adjustments other than pH. I find that I need about double the suggested HCl to lower the pH by a certain amount than what PoolMath suggests.
My chemistry is as follows:
Temp 18 degrees C
My only idea is that there is a buffer in the pool that I am not aware of. I have not added borates to the pool, and I doubt the previous owner would have. Do you have any alternative explanations or suggestions of the buffer?