When exporting double values no value type is set to the cell. When a string value is exported the cell value type is set to Text. After setting string value the valueType is set to Text and when setting again value to double the type remains the same. When exporting double values no value type is set to the cell. When a string value is exported the cell value type is set to Text. After setting string value the valueType is set to Text and when setting again value to double the type remains the same. Workaround: Just set the value once and do not change it.