Shepherd - 4:40 pm on Jan 29, 2013 (gmt 0)

and no, people will generally link to the whole URL unless they are very web savvy and know that the variables are simply analytics information.

Both pages are live with the same content:
example.com/widget/?pk_campaign=adwords&pk_kwd=keyword
example.com/widget/

More people visit example.com/widget/ therefore it is more likely to be linked to or bookmarked than example.com/widget/?pk_campaign=adwords&pk_kwd=keyword.

example.com/widget/ is linked to inter-site so crawling the site should provide them with that page over the adwords version.

Lots of ways they could find that URL

yep, but many times the simplest explanation is to correct one, here the simplest explanation is that bing discovered the url via adwords and based on the url variables it was through google search or partners, not a content ad.