It seems they made a bug somewhere in the driver. My ATIK 490ex does the same thing - it reports exactly one line and one column less than it should. (I've just made a post about it and then I found out you are reporting the same problem with 460ex driver).
To verify: open the ASCOM Platform 6 -> Tools->ASCOM Diagnostics, then select "Choose Device" - > "Choose and Connect to Device", next Select Device Type, click "Choose", select your camera, click "OK" and then click the "Connect" button. It will display it's properties with the invalid size.