I wouldn't worry about it. With the scope it is very simple due to the pure optical construction. Telephotos and zooms get so complicated that all kinds of weird things go on in the optical path. But there seems to be a general rule or two. Focal length is only more or less what is printed on the barrel at infinity, and the closer you focus, the shorter the focal length becomes. That does not seem to be the case with a scope, so your original question was actually backwards. The zoom is the culprit and not the extension tubes on the scope.