Why does VP Sales get paid more than VP Engineering?

They are similar high level positions, just in different departments, but it's pretty common to see VP Sales make at least 20% more than VP Engineering. If software developers really are in short supply and really good ones even more scarce (the type of person that could fill a VP Engineering role), then what explains this disparity?

Not to pick on GeoPalz, but they are one data point that I could easily find that shows this income disparity is: http://www.builtindenver.com/external/node/66881. They have openings for both VP Engineering ($85k-$115k) and VP Sales ($100k-150k)