When the column width is not an integer number, but a decimal fraction one, the width which is exported is not calculated correctly. 

For example, if there is a cell with some text and its column's width is autofit, it's exported with different actual width.

In addition, if the exported xlsx file is imported back in the Spreadsheet, the column width is a bit different from the one which was exported (only for the first export-import roundtrip).