"why do I get two columns of "differences", instead of only one ?"
The DIFF documentation explains this. The documentation states "diff(X) calculates differences between adjacent elements of X along the first array dimension whose size does not equal 1..." and it then procedes to give a detailed explanation of the returned sizes for vectors and matrices: "If X is a nonempty, nonvector p-by-m matrix, then Y = diff(X) returns a matrix of size (p-1)-by-m, whose elements are the differences between the rows of X"
So because you did not specify the dimension argument and the first dimension of your matrix is non-scalar then DIFF operates along the first dimension. So far everything you show is exactly as documented and expected.
