Grid header and body columns different widths

I have a data grid which is setting different column widths for the header and body. Some of the body cells have text which exceeds the standard width for a cell so is resizing the body columns and not the header columns.

I have looked through the API to find a function which does not resize the columns no matter what is in each cell but nothing has worked.

I have attached a small snippet of the grid to demonstrate.

Any ideas?

Please, check that you have a latest dhtmlx version in case of using the IE10-11 browsers,
Check you if you open your page with the valid document mode/browser mode
Check your doctype.

If the issue still occurs, please, provide with a complete demo, where the issue can be reproduced locally.
Here you can find a tutorial about creating a complete demo: … leted_demo