1) Walmart doesn't sell Pratt & Lambert paint. 2) Walmart sells a Sherwin Williams 'variety' of paint under the Walmart label. (Pratt & Lambert, by the way, was bought by Sherwin Williams in '96, I believe.) 3) Comparing the Walmart brand-label of Sherwin Williams and the Sherwin Williams Paint Store (SWPS) paint is like comparing apples and oranges.
If the paint sold at Walmart was the 'same' as the paint sold at the SWPS, then it wouldn't matter which store you went to. Also, if they were the 'same' you can bet every dollar in my wallet that every owner/operator of every SWPS would be either up in arms or out-of-business. I know that Sherwin Williams wouldn't risk putting their paint stores out of business. Hence, the apples vs. oranges. Paint consists of ingredients such as solids and binders just to name a couple. And when it comes to solids, even though the packaging may show the same solids, they are different in quality in and of themselves. So, to wrap this up, I will stand by my statement in this case. You get what you pay for.
If you have any particular questions, you can call Mr. Bill Allman at Sherwin Williams 216-566-2276 for the same explanation, and more. Bottom line, if you're happy with paying whatever you pay for paint at Walmart, then I'm happy too. Yes, it's your money; no, I don't have to live with it. My best to ya.