Pools don't always react the way you might expect. Simply retest, recalculate and try again. When making large changes it is often best to go in several steps, adding only part of the calculated amount, and retest after each addition to see where you are. Over time you will get a feel for how your particular pool reacts to the chemicals you typically use and be able to adjust the amounts based on experience.
There are several possible causes for inacruracies in the calculations of how much to add. First, the usual PH test will show PH values below 6.8 as 6.8. So your PH could have been quite low and you wouldn't be able to tell just how low. Second, PH changes depend on many different factors. It can take more or less acid/base to change the PH at extremes of PH or with different alkalinity levels. Only Richard's Pool Equations spreadsheet fully models some of the more complex factors, and it is not designed for ease of use. Third, there are always measurement uncertianties, the PH test can read slightly wrong in either direction, you may not know the volume of your pool exactly, the potency of the chemicals can vary, etc so the amounts calculated will never be exact.