And "production date" can often precede "release date" by a wide margin. I have a completely-production, purchased-at-retail original Macintosh, yet its serial number says it was made in late 1983!

Not only do they have to get production started far enough in advance of release to have sufficient systems in the distribution channel to meet demand, but as others say, they need to have "production quality" systems created simply to test the production system. Rather than throw those systems away, it makes perfect sense to send them out as pre-distribution samples.