[stack] a doubt on app1

what is the difference between app1 and i in joy?

The documentation on app1 says
app1 	X [P] -> R 
	Executes P, pushes result R on stack without X.

so executing 
1 2 3 4 5 6 [1 2] app1
should have given me 1 2 3 4 5 1 2 but it gives 1 2 3 4 5 6 1 2
which seems similar to just 
1 2 3 4 5 6 [1 2] i

I checked out what happens if the quote consumes arguments from the 
stack.

1 2 3 4 5 6 [+ +] app1
gives 1 2 3 15, so it is not stack invariant either.
1 2 3 4 5 6 [+ +] i
gives 1 2 3 15 too.

The implementations of both
PRIVATE void i_()
{
    ONEPARAM("i");
    ONEQUOTE("i");
    SAVESTACK;
    POP(stk);
    exeterm(SAVED1->u.lis);
    POP(dump);
}
PRIVATE void app1_()
{
    TWOPARAMS("app1");
    ONEQUOTE("app1");
    SAVESTACK;
    POP(stk);
    exeterm(SAVED1->u.lis);
    POP(dump);
}

does not seem to have any difference except for the assertion of
two params at the begining.

Could some one please clarify?
                                  rahul