The comma is based on the decimal separator set in your system settings in excel. If you set the type to number in the writer you should see a consistent output for all three attributes. What's a bit odd is that the setting of string in the excel writer doesn't seems to give inconsistent results depending on the fme data type when viewed in excel
I have test it further and the fme interpreted it wrong or rather it wound't transform correct in the excelwriter. It should be a string. A workaround is the StringFormatter with the "Format String option" of .2f.