OK, I had missed that you had diluted the water to get the CYA lower. Well it's still higher than I'd expect at 80 ppm CYA but with the hot Arizona sun and hotter water temperatures the losses will be on the high side, but I would have thought perhaps 3-4 but you're seeing 4-5.
Yes, the bleach might be lower than you expect and that could happen if it was not stored in a room temperature environment.
This link shows your average temperature this time of year with a daytime high of 104ºF and a nighttime low of 82ºF. As shown in
this post, 8.25% bleach at the weighted average temperature of 97ºF would lose strength to around 7.5% after 1 week, less than 7% after 2 weeks, and around 6% after 1 month. That could turn a 3 ppm FC loss into a 4+ ppm FC loss so maybe that explains it. If the pH was higher, that wouldn't have helped either since hypochlorite ion breaks down from sunlight much faster than hypochlorous acid (though most loss is from chlorine bound to CYA even though it's slower per bound FC, but there is so much more FC that is bound than unbound).
