FORALL vs. nested DO construct / Compare - Contrast

I have code which has the following block:

FORALL(I = 1:II, J = 1:JJ, K = 1:KK, NULL(I, J, K) == 1)
    V(I, J, K) = FRAC(I, J, K) * DX(I, J, K) * DY(I, J, K) * DZ(I, J, K)
    VN(I, J, K) = DX(I, J, K) * DY(I, J, K) * DZ(I, J, K)
END FORALL

In compiling / running the code after turning off my StackReserveSize number, I was noting this code was eating up stack.  So, I replaced it with

DO K = 1, KK
    DO J = 1, JJ
        DO I = 1, II
            IF (NULL(I, J, K) == 1) THEN
                V(I, J, K) = FRAC(I, J, K) * DX(I, J, K) * DY(I, J, K) * DZ(I, J, K)
                VN(I, J, K) = DX(I, J, K) * DY(I, J, K) * DZ(I, J, K)
            END IF
        END DO
    END DO
END DO

The code no longer gave me stack overflow errors, but it ran differently.  How can that be?

Perplexed