You aren't seeing the entire picture, though, so it makes it hard to understand the efficiency calculation.

How full are those Amazon trucks, and how many deliveries are they each making on their route? If those Amazon trucks are all full, and are making deliveries constantly along their routes, than more trucks doesn't mean less efficiency.

They aren't optimizing for "fewest truck trips to the block", they are optimizing for total cost. As long as we price in all the externalities properly (which we don't, but we could and should), then Amazon is going to be strongly incentivized to create the most efficient delivery schedule.

That may include many trucks running to the same location, or it may not. You can't tell which will be most resource efficient just by observing.