You need to understand that the pH test is "ONLY" one tool to help you learn what foods to avoid eating and to provide you with some guidelines to work with!
Once you get a pH test, then this does not tell you what may be causing it to be as it is!
This is only one part of the puzzle!
To understand all of the reasons why it may be of a certain test, then you must learn how to test to figure out what is in the sample to make the test what it may be!
Now to your question about why when you may have a high pH test, may the taking of Alka Seltzer may provide you more relief!
The answer may be simple, but the understanding of it may take some time to come to understand!
When you get some more relief from taking Alka Seltzer and your pH is already up by your test, this may mean that you have your overall body in much better shape, but you may still have parts of your body which may still be Too Acidic, which may be what is causing you problems still, and by taking the Alka Seltzer, this may provide that little extra Alkalizing effect which that organ of the body may need which may be still having problems , which is having the Brain Fog or Aches and Pains etc!
You need to understand that "IF" the body is in perfect shape, and when one organ of the body may start to get sick, that the overall pH may stay up as long as the body may have enough Reserves or "IF" the body may be getting enough Alkaline Minerals to buffer most of the Acids, but when one organ of the body gets Sick enough, then this starts to overload the whole body to the point which it then may effect the overall pH condition of the whole body!
In other words there may be stages which may exist whereas "IF" certain organs may become sick, but you have enough Alkaline Mineral Reserves, that your pH alone may not change too much, until the problem gets out of Hand , so to speak!