As you zoom in on a second focal plane scope the aim point changes because the reticle is not magnifyed. So as your target appears larger the more you zoom in the distance between the aim points, mils MOA doesn't change. So if you look at FFP scopes the cross and mils get larger the more you zoom so your adjustment for hold over, under and wind never change no matter what your zoom. Also makes making you range card essay and makes you more confident on shot placement
Glynn