You are missing the point. Sure you can charge from 90% to 100%.I connect the Battery Pack and then tap on low power mode from control center, and then tap on charge pass 90%. It then charges the battery to 100%. I have 2 iPhone Airs that I do this with daily. I know what is happening and don't need a forum member telling me it is impossible.
The point is the actual external magsafe battery is (from a capacity standpoint) sized the same as the internal one. The exchange of power from the magsafe battery to the internal one isn't perfect, there are losses, so it's impossible for the external magsafe battery to charge the internal battery from empty (0%) to full (100%). Physics don't allow it to happen.