Hi, ok, so take htis for example
The right column is a fluid width (it just has a left margin that equates to the left columns width, just as you had the margin)
In that case (if the left margin is set on the right column+ it isn't floated) then it will slide around the float and allow it to expand to the entire parents width. Quite a nice effect :).
That's normally how a 2column layout goes (that right column fluid layout anyway, lets keep it simple). So to keep the right column fluid a left margin is set and thus it will stay away from the left column and just continue to take up the remaining width. However you set a width thus making that nice effect very ruined. Thus since you had a width, we couldn't do that effect. I considered hte possibility you wanted the width more then anything, so I removed the left margin and floated it so it was just 2 floats side by side.
I hope you understand lol :). I tried to be concise and clear.