Where there's an active selection over the entire canvas you are unable to Contract your slection.
Doesn't matter how you select the whole canvas - whether a Select > All, or simply by dragging over everything.
This has been an issue in PS since I can remember. Not sure if CS6 retains the issue. I'm guessing YES.
They did it on purpose. I liked the way it was in Ps CS3 too.
And yes, it's the same in Ps CS6. Contract isn't even available when the entire canvas is selected.
I too think it is wrong. There seems to be no good reason selecting right to the edge of the displayed canvas should be an exception, but there are cases where Adobe thinks this is the better way.
You might find some enlightening info in this old thread: http://forums.adobe.com/message/3725829
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