ToS (type of service) is a byte within the IP packet. Various RFCs over the years have defined what its bit are to be used for. DSCP is one of the later RFCs which redefines what some of the ToS bits represent, for DSCP the left 6 bits.
Your question about DSCP using more bits is true if compared to how bits were used to support IP Precedence (the left most 3 bits), but not true with regard to ToS as a whole, because usage of the next 3 left most bits, after IP Precedence, were also defined in one of the RFCs.
