Why do some mergefields have quotation marks around their names, where others don't.

For example:
{ MERGEFIELD "Gadv_Date" \@ "dd-MMM" } = 13-Jun
{ MERGEFIELD "Ptimec" } = <4:45 PM
{ MERGEFIELD SubRespStart \@ "h:mm am/pm" } =0.5625

but ...

{ MERGEFIELD "close_date" \@ "dd-MMM" } = 41438