Keep in mind that bleach at Sams or Walmart is likely to be fresher, and cooler, than bleach at a pool store. Take a look at their tank -- if its black poly, in the Arizona sun, it's VERY unlikely that it arrives at your pool at the rated strength.

Another test is to 'take the temperature' of the bleach just after buying it. If it's above 90 degrees, it may be 4% bleach.

But the best test -- if you have a K2006 -- is to dose the pool with equal doses of the various bleaches, in the late PM and test the pool FC level just before, and an hour after, dosing.