Isn't it because the first 5 planes are delivered immediately?
I once ordered 20 B737MAX9, and the first five came immediately, but the next aircraft came significantly after some time. So, if I calculated right, the 5 aircrafts came immediately, but the 6th aircraft came as if the 5 aircrafts were delivered on normal schedule. So the 6th was delivered lead time + 5 build time after the order. If this is right, ordering 5 for immediate delivery and ordering the later part of the aircraft after the delivery will deliver the aircraft faster.
Sfk is right: if you have aircraft that can be delivered immediately upon order, the lead/build time would register as 0. But we need the absolute confirmation of Unk on this to be completely sure.