Strange column width behaviour when sidebar activated

I’m a bit lost when it comes to setting column widths when using a side bar. Basically, I cannot set a succession of columns to occupy the whole space left beside the sidebar.
Following are three examples with screenshots and related code used:

Example 1 (first attachment):

Is generated by the following code:
[column size=”col-2″]
<h3>Materials used</h3>
…[/list]
[/column]
[infobox align=”right” size= “col-6″ closable=”false” color=”green” last=”true”]…[/infobox]

Example 2 (second attachment):

is generated by the following code:
[column size=”col-4″]
<h3>Materials used</h3>
…[/list]
[/column]
[infobox align=”right” size= “col-5″ closable=”false” color=”green” last=”true”]…[/infobox]

And if I try to widen the content it is then wrapping (third attachment):

which is generated by the following code:
[column size=”col-5″]
<h3>Materials used</h3>
…[/list]
[/column]
[infobox align=”right” size= “col-5″ closable=”false” color=”green” last=”true”]…[/infobox]

Can you please help me out of this?