In your example, your result saves every 5-th row of the input matrix to the output matrix.
Try this:
M = [(380:400)' randi(9, size((380:400)'))] % Create Matrix
Out = M(1:5:end,:)
If you want the result to have every 3-rd row, do this:
Out = M(1:3:end,:)
EDIT — Clarifying the discrepancy.
