Here's a big azz wrench to throw in the works...

I just ran a 'Carl' on the jug of Walmart 4%.

1ml of Walmart 4% in my little 10liter 'pool' tests as 12.6ppmFC
(baseline tap water was 1.2ppmFC and sample read as 14ppmFC)

I'm fairly certain I didn't mess up. I will dump my 'pool' back into the 1liter measuring cup I used to fill it...in case I mis-counted. (even then...such a massive difference for +/- 1liter error?!?!?). I am 100% certain on the 1ml bleach sample. I ovefilled the syringe, bled the air, pushed out to exactly the 1ml line. I rinsed the syringe outside, then dumped it & flushed it. I stirred well, waited 10mins, re-stirred and multiple-flushed then pulled a 10ml sample.

If this method kinda points to %conc, then Walmart's supplier is pulling a fast one...to the consumer's benefit...which is w-a-y too suspicious. But...maybe it costs that particular supplier too much to dilute it down to 4%. -or- the supplier-du-jour only has 12% in their tanks...but, sometimes the other suppliers only have 4%...so Walmart has to mark for the least %conc. No doubt they have multiple sources for their store brand stuff. But I'd have to random sample all over the place to try and nail it down...forget it.

Anybody have anything to add?

UPDATE:
I just emptied 11 liters from the bucket. <sigh> That throws the math off...but that means the '%conc' must be even higher than 12%-ish. I just expanded the whole: 1ml in 11liters = 12.6ppm...up to my pool volume 20062liters for 1ppm change. I'm going to chuck 145ml of this stuff in and see what happens. My pool is 4ppm now. If it goes up to 5ppm, I guess it must be way more than 4%. True...I am only +/-0.5ppm resolution...but a little trinkle of 4% can't be worth much...the odds are it won't push me above 4ppm. We'll see....