There are two "layers" of formatting on a Filemaker Field. You can select text inside a field and apply a text style to it or you can select the field object while in layout mode and specify a text style for the field object on that one layout. Formatting of the data inside the field supersedes the format applied to the field object.
In non-global fields, this means that if you select text inside a field and format it, the text in this field for that one record has been formatted. The format selected does not affect that shown in other records. Styled text can also be pasted into a field and you get the same effect.
With a global field, you only get one instance of the data and so the formatting appears to be applied in a more universal fashion, but the formatting of the text in the field still supersedes the formatting specified for the field object itself.