i hate those testers.
use a digital tester to check your runoff.
that's how you really figure out what the ph of your soil is.
honestly, i've fought acidic soil before and it's as easy as watering with slightly basified (7.7) nutrient solution.
it takes a few waterings, but it works.
i have never seen the ph in soil swing like you are describing.
soil is a great ph buffer and as i said before, and significant change usually takes a few waterings.
your tester is probably just inaccurate.
or it could be that when you test the soil right after watering, the tester reads more of the acidic water than the basified soil.
then as soon as the water has drained, all it reads is the basified soil.
test your runoff.
its the only way to really tell..