The extra row and column are added for the data structure information. The -inf in the bottom right means the matrix is a SYSTEM matrix. The number in the top right denotes the number of system state. That is, no matter which function you use, if the function output is SYSTEM matrix, there will always have extra row and column.
For example, if we have a system y=(1/s)*u, then we can get the state-space model x'=Ax+Bu y=Cx+Du where A=0,B=1,C=1,D=0. It is easy to see that the model only have one system state, and the result of "nd2sys" should be [A B 1;C D 0;0 0 -Inf]