I think it's because surf doesn't render the last row or column of the matrix (so that the surface doesn't actually go all the way to x=1 but stops just before).
The problem:
M = magic(4) % 4-by-4 matrix
surf(M) % only 3-by-3 is shown
view(2)
colorbar()
To correct that, you can replicate the last row and column of your matrix, as in:
M = magic(4);
M = M([1:end end],[1:end end]) % 5-by-5 with last row and column repeated
surf(M) % 4-by-4 is shown
view(2)
colorbar()
