Well, we select on them differently. The first fish back are the ones that get to reproduce. A hatchery chinook does not need to travel hundreds of miles up a river and does not need to defend a redd, so it can put a lot more into egg production and less into commuting. That means it can be a smaller fish that comes back early. It doesn't face the risk of predation that it would if it remained at sea for a couple more years. Chinook happen to be very plastic, and they change with changing conditions. The rules for surviving the hatchery life cycle are different from those of the wild-run cycle.
On October 30th, 2018. See this statement in context.