Crew members.
When the 1-11 was certified in the USA all aircraft with a max weight over 80,000 pounds had to have 3 crew members. That is why the 400 srs. (USA)was 80,000 pounds and the 300 srs. (UK) was 84,000 pounds or there abouts. Both variants were almost identical.
By the time the Boeing 737 came along the rules had changed.