You could try what PharmerAntony suggests and based on our typical experience you will probably find that 2 or maybe 3 are close on a few readings, and significantly different on others and 1 or 2 are so far off you will wonder if you have someone else's results. Or take a moment and ask yourself from a business model point of view, what incentive does the pool store have to get the often free test right and minimize your chemical usage? Afterall what do most customers do when the hundreds of dollars worth of pool chemicals fail to solve the problem, or only temporarily solves it while creating 2 new ones. They faithfully bring in another sample, see the "infallible" computer print outs with pre-printed advice, and pull out the credit card once again to buy this and that magic in a bottle. Don't blame the kid working at the counter, much of the time he is just doing as he was trained, remember most smaller pool stores get their training from the chemical supplier reps, and the larger ones get it from the corporate training departments.
Or lets take this another way, you own a small pool store, even assuming you got into the business wanting to help people and make some money, so like many small businesses you feel like you are being squeezed from all sides, and are not sure how you are going to make your note payment, rent payment, etc. and one day after trying to do what is right for the customers by giving them the best advice you know, which may not be all that great, a sales rep from big corporate pool chemical supplier walks in, and ask why your sales volume is so low, and he says he has just the tool to help, for only a small investment of a couple of thousand dollars, which will include some special rebates on chemicals purchased in the future he will install this computerized pool water tester, it will look all fancy and scientific and print out exact advice including their brand name product, like Super Stuff #20 for when the test says you need to raise TA, which is just 99% baking soda and 1% special binders to make it looks like flakes not regular baking soda that sells for 1/5th the price. (and hey who cares if it is a bit off, they will come back and buy whatever else ti tells them to correct the over/under shoot, side effect, etc) A month later business is booming, customer's are walking out spending $300 instead of $75, and all is good, afterall if these people can afford a $40,000 pool what harm is there in taking an extra hundred or two from them each time they come in the store. After a while you get used to this, and want a more profit, but when it comes down to it there is only so much people will pay for baking soda, and enough of them remember enough high school chemistry to know when they read the label on Super Stuff #20 that Sodium Bicarbonate is just Baking Soda or something close. So you move on to fancier testing for things like Phosphates, oooh that sounds bad, and everyone knows Phosphates are algae food, so if you can test for it and the number is anything but 0 now you get the chance to sell expensive phosphate remover which can't be bought in the grocery store with a big arm and hammer on the side. Of course somewhere along the line someone neglects to bring up that phosphates are irrelevant if you have enough chlorine in the water to kill the algae, much like it does not matter how much cheese you have in the refrigerator if there are no mice, and you keep any that might try to move in from getting to it (keep your chlorine levels right). So now business gets even better and before you know it you have gone from wondering where money for rent is going to come from to openings 3 new locations.
Ike