Hum, I am not sure I can explain this simply. It really takes "deep end" level chemistry to understand what is going on. The short answer is that the PH reads higher than actual and the PH is actually higher than you might think, both at the same time. Unless you are really into this stuff, it is best to stop here and simply take our word for it.
Read further at your own risk.
There are actually three effects, two chemical reactions in the PH indicator solution, both of which cause the test to show PH values that are higher than actual levels, and a third caused by details of how bleach affects PH that cause the PH to actually be higher than you would expect while you are shocking the pool with bleach.
At extremely high FC levels, chlorine transforms the phenol red indicator used in the PH test into chlorphenol red. Chlorphenol red is used to measure PH values between 5.2 and 6.6, so any normal pool PH level will read at the top end of the range. The top end of the chlorphenol red indicator range is similar to, though not quite the same as, the color for a PH of 8.2 using phenol red.
The Taylor PH reagent contains various chemicals that prevent the transformation to chlorphenol red from happening at intermediate FC levels. The reaction of these stabilizing chemicals with intermediate FC levels causes small PH increases that create small reading errors when FC is above 10, with the error increasing with increasing FC levels.
Then, as the FC level continues to increase, at a bit above a FC level of 20 for Taylor reagents (and at lower FC levels for other brands), the conversion to chlorphenol red starts to happen. This conversion causes a final dramatic increase in the apparent PH, completely invalidating the test result (unless you understand exactly what is happening and use the chlorphenol red color scale).
Meanwhile, bleach has a temporary effect on the PH, which we don't bother to explain very often. When you first add bleach, it causes the PH to rise. Then as the FC get consumed the PH falls again, eventually going back to almost exactly where it started. If you run at fairly stable FC levels, this effect is invisible. The PH rise from adding bleach is canceled out by the PH fall from chlorine being consumed. The two effects happen at the same average rate (because we are maintaining a constant FC level) and thus cancel each other out. Because of this canceling out, this effect is normally ignored.
When shocking, large amounts of bleach are added and the PH goes up temporarily (as long as the high FC level holds). These high PH levels are misleading, since things will go back to normal when shocking is over. If you were to reduce the PH while FC was high, the PH would go down even more when the FC level was allowed to return to normal, creating problems.
