By default, there is a default style in every feature layer in Cartographica. For simple layers (ones that have no other styles), this provides a simple way to edit the style without having to create a new style. However, in the case of complex layers, it is often completely unused. This can cause a problem with a Map Layout in that you may have an unnecessary "Other" or "Non-Matching" style in your legend.
For Cartographica 1.4+, you have control over hiding or showing each individual style in the style set, and you can name the "Non-matching" style as you wish. So, if you want features that don't match (of which there may be none) to be ignored on the map and in the legend, check the Hide these features box.
In Cartographica 1.2, you can remove the "Non-matching" or "Other" style by checking the box in the Style next to Hide features that don't match.
Basically, the legend mirrors exactly what is available in the styles. So, if the default styles can be displayed, then they will be shown in the Legend, and if they are hidden, the legend will also hide them.