Quoting from the Taylor "Pool & Spa Water Chemistry - A Testing & Treatment Guide", under "TESTING pH"..."FALSE READINGS:"

high levels of chlorine (usually > 10 ppm) will quickly and completely convert phenol red into another pH indicator (chlorphenol red). This new indicator is a dark purple when the water's pH is above 6.6. Unfortunately, some pool operators mistake the purple color for dark red and think the pool water is very alkaline and wrongly add acid to the pool.

When a sanitizer level is not extreme, only some of the phenol red may convert to chlorphenol red. However, purple + orange (for example, pH 7.4) = red. This error is more subtle as no purple color is observed and the operator does not suspect that a false high pH reading has been produced. Some operators neutralize the sanitizer first by adding a drop of chlorine neutralizer (i.e., sodium thiosulfate). However, thiosulfate solutions have a high pH and, if heavily used, may cause a false higher sample pH.

It sounds like your high chlorine levels may have fooled your pool store into thinking you had higher pH, but I'm not sure why your test didn't have the same problem. Perhaps the chlorine level dropped somewhat between the two tests or the pool store pH test may not have any thiosulfate built into it the way that the Taylor test does (I think). I would probably trust your numbers.

The chlorine level should drop rather quickly if you keep your pool uncovered during the day and exposed to sunlight. It may lose at least a third of its chlorine in a day this time of year (up to half during peak summer).

Richard