In Arma 3 v1.55.133361, a new syntax for getPos command has been recently introduced:
_return = _originalPosition getPos [_distance, _heading]
OBS:
The returned Z value (_return select 2) is always 0.
EXP:
The return Z value should be equal to the Z value of _originalPosition.
(getPosASL player) getPos [20, 0]
and
[(getPosASL player), 20, 0] call BIS_fnc_relPos
This is a design decision, the z will always be z of the ground for convenience. From BIKI page:
"Return Value:
Array - format [x,y,z], where z is land surface in format PositionAGL"
Same with getRelPos. When over water it will return negative z. Makes it easier to calculate an offset if needed.
One other reason is that BIS_fnc_relPos z is treated as AGL even if you supply different format, and as a result the new position may not be exactly horizontally away from origin anyway.
Hmmm, I guess I can see why that is generally the most useful return, as ground level is most useful most of the time.
I can use other methods to achieve a true horz offset.