See this fiddle: http://jsfiddle.net/JaapM/PtQB7/
The third column has values, so a DropDownList control is generated for it.
When you click in the first column, the editor is show. Then tab to the second column. The editor is displayed also.
Then tab to the third column. The editor (dropdown) is not displayed. It is just a focused cell. When pressing Enter or F2, the dropdown is displayed, but the table cell in the background this has the focus. Also you can navigate away from the cell with the arrow keys, this is not possible in the other columns.