The problem is that zeners aren’t an ideal device, the graph of voltage versus current doesn’t make a sharp, right-angle corner: the voltage rises with current until it approaches the rated current, at which point (known as the “knee”), it curves over into the constant voltage region, where the voltage doesn’t vary a whole lot from there to max current. There’s a passable explanation (and a useful graph) here: https://www.electrical4u.com/zener-diod … regulator/