Why do I have to hit tab twice to Alt+Tab away from Microsoft Excel?

In other apps, when I hit Alt+Tab it switches to the previous app. Before I actually release the Alt key I can see a list of windows as well and the previous windows is selected. But in Excel 2010 (Professional Plus, if that makes any difference) hitting Alt+Tab and not releasing Alt shows Excel still selected, and in any case I have to hit Tab an extra time to switch to the previous application. (Alt+Tab+Tab) Furthermore, the iconized view showing my spreadsheet only takes up about 1/3 of the height available.

Needless to say this throws off my workflow because switching between Excel and another application is inconsistent, requiring an extra keystroke to switch away but not to switch back. Is this a bug in Excel, or is there some option that I need to change to get the default behavior? What's going on here?

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