Normally, data has no style attributes and is displayed using the font, size etc. specified for the field in Layout mode (you can even put two instances of the same field on the same layout and display each in a different style).
However, it is possible for data to have styling of its own - this will happen if you apply a style in Browse mode, or use one of the text formatting functions, or paste styled text from another application. In such case, the data-level style will override the layout-level settings.
You can apply formatting to the text in the field or apply formatting to the field itself and pasted text can include text formatting.
Any formatting applied directly to text will over-ride formats applied to the field.
Use the following calculation to strip all formatting from the text: TextFormatRemove ( text )
If you set this up as an an auto-entered calculation, formats will be stripped automatically when they are input by the user.
You can then apply formats to the field (select it in layout mode and choose text format) to give a consistent text format to the field for just the current layout.