Cyclocross frame size vs. road bike frame size
Bicycle sizing may vary by bicycle type, region, manufacturer, and material. And measurements typically are of a length of the frame (usually bottom bracket to top tube: How do I measure the size of my bicycle frame?). So it would not be surprising if this statement was true.

Depending on the geometry of the frames, two bikes may have different "sizes" but actually fit the same size rider well. It is possible that cyclocross frames tend to have a smaller measured sized.

Here is a post that goes in depth... and is ultimately inconclusive: http://www.bikeforums.net/archive/index.php/t-138316.html