I think a lot of people adapt the fink plan to suit themselves. Even if it's just by not following every swim set to the letter it's an adaptation. Fink will have had average speeds in mind for his sessions so some people will need to add extra time on, maybe the plan would have been better written in miles and followers take as long as they take
The sprint tri's you may have done might have had quiet roads, I'd hate to be racing round some of the routes I've done much after lunch time!
I reckon it's a combination of giving the pools back in time for the Sunday rush, avoiding traffic and getting everything done and dusted in time to be at en pub for Sunday roast