AT One column doesn't work well with minipanels

I created a Panel page using the Layout "AT One column".

I then put a mini panel in that 1 column. The mini panel used the Layout "AT Three column 3x33" and in that panel there are 3 panes arranged horizontally.

On a standard screen (PC), the mini column was rearranged into a vertical column at the far left hand side of the screen, as per my theme settings for Table and Smalltouch screens. It is as though the "AT One column" layout fooled my PC into thinking it was a tablet or Smalltouch screen.

Custom content placed into panes in the "AT One column" page spanned the full width of the screen as expected and desired.