XSSFSheet has this code in the write method:

            if(row.getLastCellNum() != -1) {
                maxCell = Math.max(maxCell, row.getLastCellNum());
...

        if(minCell != Integer.MAX_VALUE) {
            String ref = new CellRangeAddress(getFirstRowNum(), getLastRowNum(), minCell, maxCell).formatAsString();
            if(worksheet.isSetDimension()) {
                worksheet.getDimension().setRef(ref);
            } else {
                worksheet.addNewDimension().setRef(ref);
            }
        }

Arguments to the CellRangeAddress are zero-based:

	/**
	 * Creates new cell range. Indexes are zero-based.

However row.getLastCellNum() returns the zero-based last cell number PLUS ONE:

     * @return short representing the last logical cell in the row <b>PLUS ONE</b>,
     *   or -1 if the row does not contain any cells.
     */

As a result the dimension in the worksheet has the last column number off by one.  A single column sheet with five rows is written with 

<dimension ref="A1:B5"/>

instead of

<dimension ref="A1:A5"/>