In my test... I dropped a ROW directly into a SECTION.
No classes on the SECTION or ROW.
It appears... the COLUMN of a ROW has a built-in LEFT and RIGHT MARGIN of 10 pixels.
You can see this adding a ROW - then adding a class to the one of the columns.
The 10 pixel left / right MARGIN will appear. The MARGIN is always there - but you cannot the values until you class the COLUMN.
If you drop a DIV into COLUMN 2... the DIV is incorrectly positioned inside the COLUMN... about 20 pixels down.
If you class the DIV... and add a ZERO (0) top MARGIN to the DIV... the problem doesn't do away.
If you change the DIV's top MARGIN to 10... the DIV is repositioned correctly (occupying / moving to - the top of the COLUMN)... but the MARGIN is about 20 pixels. NOT 10. The only way to fix this is change the top MARGIN of the DIV to 1 pixel. You will always be 1 pixel off though.
If you drop a ROW directly into a SECTION.
Then drop a DIV into COLUMN 2 of the ROW...
The DIV is positioned about 10 or 20 pixels below the COLUMN top.
If you create a class for the DIV - and add a 10 pixel margin...
the DIV is repositioned (correctly) to the top (zero 0) of the COLUMN.