I for one thank you for the effort. I have tried to use Amazons' default (pick a category, enter 10% off, 20% off, etc) and it is usually hit or miss if it actually works.
As for the one you made, I punched in a maker (Dewalt) and then tried the various percentages. I would imagine it is Amazons' screwed up "discounting" pattern again, but if you actually look at the prices of say the 70% search, they are really not 70%, more like 50-60%.

Again, I would venture to say that is Amazons' flawed system, not your search parameters.
I think the "commonly searched" version is a really cool way to do a search in a minute or two, instead of having to manually type it all in on their site.
Good job.
ETA: OK, Went thru the quick search version again and have come to the conclusion that Amazons' "discount" search is SERIOUSLY flawed. Pick any brand from the list, use 70, 80, 90% and watch the results. Freakin' hilarious.
Example:
http://www.amazon.com/gp/search/?ur...p_8=90-&tag=toolsearch-20&index=tools&p_76=1-
http://www.amazon.com/dp/B0001NQQCM/?tag=atomicindus08-20
I didn't realize that 90% off of $40.38 is $29.88!! Must be that "new" math.