Liberation fonts

Quote from fedorahosted.org/liberation-fonts/ : There are three sets: Sans (a substitute for Arial, Albany, Helvetica, Nimbus Sans L, and Bitstream Vera Sans), Serif (a substitute for Times New Roman, Thorndale, Nimbus Roman, and Bitstream Vera Serif) and Mono (a substitute for Courier New, Cumberland, Courier, Nimbus Mono L, and Bitstream Vera Sans Mono). How can Mono substitute for both Courier New and Bitstream Vera Sans Mono, when one has serif and the other not? It seems very like that set of attributes that are the generic font names, serif, sans-serif, cursive, fantasy, and monospace. They are only attributes, not disjoint sets. Besides Courier New that has serifs and pitch, and Bitstream Vera Sans Mono, which has pitch but not serifs, there is an old Selectric typewriter face called Script 12 pitch, which is also cursive. "Serif" and "sans-serif" are disjoint, and I would vouch for nothing with "fantasy", but "monospaced" is not disjoint with three of them--is "cursive" disjoint with "serif" and "sans-serif"?