Overlay not lining up with custom grid

Hi,

I have managed to get a custom grid in (using 978gs for 'normal' and 748gs for 'narrow').

However, my grid overlay doesn't line up. Ignoring the narrow size, for normal, I resized the overlay grid png to 84px across. This is 54px column width plus a gutter of 30px.

With the 960 grid, the overlay lines up perfectly to the edges of the debugging blocks. Not on mine though! Any ideas??

http://dl.dropbox.com/u/63070476/Screenshots/Screen%20Shot%202012-10-01%...

I know the grid is correct as the orange area is 1008px across (total size for the 978 grid). The green area is 978px across.

Everything is good, apart from the overlay.

each column (the pink bit) is showing a width of 64px, however, my columns are 54px each. The gutter is 30px. Margin is 15px on the left and on the right. Kind of looks like the overlay thinks my margins are 10px.

Thanks.

Sam.