In short, Carloman & this brother inherited titles from Pepin the Short
[DOC] [TLE] CATHOLIC ENCYCLOPEDIA: Charlemagne - New Advent [PAR] At the time of Charles' birth, his father, Pepin the Short, Mayor of the Palace, of 
the ... the last Merovingian King of the Franks; but this modest title implied that 
real ... the effect of the act of partition under which Charles and Carloman 
inherited their ...