These "mirror" (MITM) pages outrank the authoritative sites for many projects because Sourceforge has been around for 10+ years and has superior trust/backlink profiles compared to the newer author-blessed sites which presently host the software. Gimp is actually fortunate in this regard -- gimp.org is stickied to the top spot when searching [gimp] and Sourceforge floats around #8 or so.
Sourceforge should get hit with Google's standard penalty, which is "we smite your rankings with the hammer of an avenging god." Minimally, Google should at least tighten up their enforcement of AdWords policies. Their "installers" are per-se violations of the Unwanted Software Policy (http://www.google.com/about/company/unwanted-software-policy...).
How about it, resident Googlers?