This have been discussed many times before, but in a nut shell
The max sensor size on 5d2 is 1880 not 1920 like 5d3 for video
5d2 use line skipping and pixel binding to get that size,
Where the 5d3 have a completely redesign sensor that uses all lines for video
No pixel binding or line skipping, that's way there is no moiré or aliasing to speak of .
There again that's why the 5d3 have faster CPU, it has to do more etc....
Edit: In Crop mode the window has over 2000 plus lines so you can get 1920x1080 or there about.
So you are using All the lines with out line skipping etc..
So just like it says crop mode is really taking a section of the sensor
