The parser for make.conf does not work well with quotation marks. If the value has multiple entries, using quotation marks prevents the file from being parsed properly.
For example, if you enter
OPTIONS_UNSET="DOCS EXAMPLES NLS"
it is not parsed properly.
If you enter
everything works fine.
If the parser was changed so that quotation marks are parsed well, then the sysrc(8) utility can be used to create and modify make.conf.