jQuery Grid Plugin - jqGrid

I have a jqGrid with just 2 columns.
The problem is that size of columns is not the same that I've set in their "Width" property.
E.g. I have specified

Column1 width = 150px, Column2 width: 80px

But when I load jqGrid, I see that sizes of respective column and data cells are slightly different.
Indeed, when I look up width values in FireBug I can see

Header1 width = 147.4px, Header2 width: 78.8px (!!!)
Column1 width = 148.33px, Column2 width: 77.667px (!!!)

So their sum is the same is above, but item values are slightly different.
If I add a couple more columns, this distortion becomes more visible with each next column.

I've made sure, that shrinkToFit option = false, so that only real pixel values will be used, not proportions.
But this behaviour persists both with shrinkToFit option = true, but disproportion between headers/data is slighly different.
So even with shrinkToFit set to false, where I am supposed to have total control over column/header width, "someone" changes my values. I've made sure that no external css is included, so no external incluences can be considered.
Also, columns names/values are short and fit nicely inside of columns, they don't "expand them".

What can be the problem? Is there any option I missed or misunderstand?

The grid options code is smth like this:

url: 'myurl.com',
            datatype: 'json',
            mtype: 'GET',
            height: "auto",
            treeGrid: true, treeGridModel : 'adjacency', ExpandColumn : 'TreeField',
            loadonce: false,
            prmNames: {sort: "sidx2″, order: "sord2″},
            shrinkToFit: false,
            viewrecords: true,
            imgpath: 'themes/basic/images',
            caption: 'MyGrid',
            rowNum: -1

Playing with   

            //autowidth: true,
            //forceFit: false,
didn't align headers with data either.

Also, would jqGrid allow if I set width values with direct jQuery calls?