But presumably that's not a critical error. When you are signalling, you aren't trying to hold the spot fixed on the 'plane. Instead you "flash" by moving the spot up and down. So vertical accuracy doesn't matter as long as you get the spot well above and well below the target.

(Flashing is both easier to do and more noticeable for rescuers. Horizontal accuracy matters for this, and the method in the diagram is designed to maximise it.)