Post order issue

Hi,
I am having an issue with post ordering when I change the grid settings.
The query post settings are set to descending and date and when the desktop grid settings are set at 3 columns all is fine, the posts are arranged 3 2 1 in a single row. If I change this to 2 columns the order changes to 3 1 in the first row and 2 in the second.
I would have expected it to be 3 2 and 1 in the second row?

Any help much appreciated!

Thanks
Chris