To reproduce:
Try setting the value in the CellForamting event. 

Workaround:
class MySpreadExportRenderer : SpreadExportRenderer
{
    public override void SetCellSelectionValue(DataType dataType, object value)
    {
        base.SetCellSelectionValue(dataType, value);
        if (dataType == DataType.Number)
        {
            CellRange range = ((CellSelection)this.GetCellSelection()).CellRanges.ElementAtOrDefault(0);
            double d;
            if (double.TryParse(Convert.ToString(value), out d))
            {
                var cell = ((Worksheet)this.GetWorksheet()).Cells[range.FromIndex.RowIndex, range.FromIndex.ColumnIndex];
                cell.SetFormat(new CellValueFormat("#,##0.00"));
                cell.SetValue(d);
            }
        }
    }
}