Your code is likely not doing what you think it is doing. The randperm(427) call gives you a permutation of the numbers 1,2,...,427. So ALL of the rows from 1-427 are selected and the rows not selected are always 428-600. From your description, it sounds like what you really wanted was to randomly select 427 out of 600. I.e., is this what you really want?
k = randperm(600,427); % <-- 427 random integers from the set 1-600 (no replacement)
xnew = b(k,:); % <-- select the random rows
m = ~ismember(1:600,k); % <-- get logical indexes of other rows
xother = b(m,:); % <-- the other rows
Another way to get the other rows per your suggestion:
xother = b;
xother(k,:) = [];
