Nancy, I don't think it has to do with distribution of chemicals in the bag - the industrial mixers they use on salt mix are pretty robust. PH vairies a lot due to CO2 in the water, though, so that may be the cause of the fluctuations you are seeing.
tfever, I suggest you get a container that holds the amount of water you are replacing. Fill it with RODI water, add an airstone for 02 or a powerhead to mix the salt (add it slowly and test often) and a heater and let it mix for a day. When all the levels are where you want them to be, do the water change.
