moves on shield do X amount of frames of shield stun
when you do an aerial on shield as early as you can in your short hop, you spend time in the air before Lcancelling. frames spent in air are frames wasted when a move does X amount of frames of shield stun
when you do an aerial as late as possible in your double jump, there are no frames spent in the air, only frames of Lcancelling
so lets say you do a move that does 15 frames of shieldstun:
you hit that move on their shield early in your short hop, and then spend 10 frames in the air and 8 frames Lcancelling
those 10 frames spent in the air are wasted
lets say you hit that same move on their shield as late as possible, and then spend 0 frames in the air and 8 frames Lcancelling
there are no frames wasted and you are +7 frames actionable compared to them
sorry if i didnt explain it well