The problem, in the steps you said you followed, is the "Print Preview" step. File >> Print Preview will show you a preview of how the entire current page will look like, when printed -- but that's actually not what you want to print.

What you want to print is only contents of Readable's overlay. To that end, please use the Print link, shown in the menu at the bottom of the overlay -- unfortunately, there is no "Print Preview" available.

The technical explanation for this is that Readable's overlay is actually an IFrame -- and browsers support printing the iframe contents as if the iframe were a window onto itself; but they will print the iframe as an element in the main page, when you're printing that instead.