I wonder if Yahoo is inflating their numbers with irrelevant document types. For example, if Yahoo counted CSS files but Google didn't, that could help explain why Yahoo on average returned fewer results even though it had indexed more pages.

This seems possible, since Yahoo reports 10,100,000 results for ["background-color"], whereas Google reports 2,390,000 results for ["background-color"].

I'm not sure. A quick look shows "inurl:default.css" doesn't seem to return any actual CSS files in Google.
This phrase search:
"background-color: rgb(180,109,104); color: rgb(104,49,45);"

will show many results in Yahoo, but none in Google... but in Yahoo, the results are also no CSS. I wonder...