The difference is that although the active ingredient is listed as being the same chemical, they are not required legally to state that the molecule is only in the same chemical family it is slightly different than Provent-A-Mite, also the binding agents that allow the chemical to dry and bond to your substrate are different. This amounts to the major difference being whether or not the pesticide is absorbed into your snakes body in large amounts or not. This is how I understand it atleast, and I understand your devils advocate point, but there is a link earlier on this post that goes into detail explaining all the differences, I would check it out, you can go cheap if you want but when it comes to your snakes health it may be a poor choice to chintz out... Also the pesticides they are selling in Walmart are probably manufactured in China, and China has a poor track record when it comes to accurately stating what is in their products, just google melamine poisoning in Chinese milk, lead and cobalt levels in Chinese made toys, and terrible quality control with Chinese made medications, etc. etc.
By the way great point Dr. Del !!!
I am no expert in how exactly Provent-A-Mite works, but I do know I have used it in the past with perfect results and no harm to my snake, and have read nothing but positive feedback on the product from experts all over.