The module does not actually record the data. That is done on Google's end. So any tracking that is not consistent is a Google side issue more than likely.
This module simply provides the ability for tracking to take place by placing the code on the page.
There will always be differences in tracking to some degree (more on the ga.js version than the urchin version) because google uses javascript to track.
For Yahoo, it should appear just like any other engine does UNLESS you are talking about "yahoo search marketing" 9as in paid search or ppc).
Google does not track that info out of the box and you have to make some adjustments (on google's end not within the module) to track that information in your Google Analytics.
Here is an article that may help you do that.
How To Track Yahoo Paid Search With Google Analitycs
Bookmarks