Hi Andrei
It appears that the issue is related to the machine I'm running it on - a 27" iMac running Yosemite 10.10.5
However, when I look at the layout on my macbook - also running 10.10.5 - the grid and layout appears correct.
FWIW, it's a 12 column grid set to half height (2:1), with an iPad Portrait layout (758px wide).