With SEO it can be very hard to be totally specific, but looking at those three pages;
The term appears something like 8 or 9 times on the page, it is spread mostly evenly across the page and half of those appearances are in bold/heading text, textbook good SEO copy.
The term appears something like 13 times and there are several instances where the term is "stuffed" in the "condition of..." paragraph and the last paragraph. OK SEO text but pushing a little too hard, the META keywords also over-use/stuff the term.
The term appears about 7 times, only two are headings and there's stuffing right at the start;
"KA36200 Square D Circuit Breaker
We stock Square D KA36200 Circuit Breakers , new, used, and obsolete - full 1-year warranty
Part: Square D KA36200 Circuit Breaker"
Schizophrenic SEO text where the word generally isn't used enough and when it is it's stuffed closely together.
I cannot guarantee these are the reasons why the rankings are as they are but in my opinion WCP needs to just balance out uses of the keyword across the whole page better, it really can be down to tiny adjustments especially with e-commerce product pages which are always going to be quite similar from site to site, I would try a few different copy/text solutions on different product pages and see which template gets the best results.