We are using igHierarchical Grid in our application .On initialization the width of the parent grid is set as 100% and we have set autofitLastColumn: false. Once this is set as false the grid header borders and grid data columns get mis aligned .Please see the screenshot.
We have 21 columns in the grid and the width of each column is set in px.
I've tested hierarchical grid with more than 20 column, width: 100% and autofitLastColumn: false. You didn't tell me the product version you're using, but I've tested with the latest one (v18.1). Another thing to note is that I saw that you're using different theme than the default one. It looks like the infragistics2012 theme, but I may be wrong. Please refer to the attached sample and modify it so that you reproduce the issue, then send it back to me.