Adding this to the bottom of your stylesheet fixes the problem:
Code:
#navColumnOne {
width: 200px !important;
}
From view source, it appears that you have in admin set #navColumnOne to a width of plain 175, and #navColumnOneWrapper to a width of 195px. CSS styling requires a unit for the dimension (width: 200 !important; doesn't work).
With the left column not having a fixed width, the margin: auto; in #navColumnOneWrapper pushes out the sides when the main column doesn't have enough solid width to contain it.