The answer is probably in what Google calls the "expected click-thru" rate and "search intent". I Googled both terms and came up with very different results.
It is likely that there are far fewer search queries for "tennis shoe". The organic results include a definition of what that is. The term is a bit ambiguous as to search intent. A search for that turns up a page with no ads in the top position and only a few on the side.
A search for "tennis shoes" turns up a full page of results of sites that sell tennis shoes, and a full complement of ads to go with it. Apparently, folks using this term are (mostly) looking for places to buy tennis shoes.
I hope this helps.
Best of Luck!
Pete has some great thoughts there!
Even though the two keywords are very similar they are different keywords and will be treated differently by our systems. The keywords will also have different performance histories on AdWords, which can have an influence on their quality score.
Hope this helps!