Very interesting!
There’s been a lot said about the lactic bit but I suspect that the 400 forces the athlete to be more even in his energy distribution over 200m.
Another thing you’ll often see along these lines is a young athlete passing through 200m in a pb on the way to a 400. They are often shocked to hear it but have trouble bettering it when they run a 200 flat and burn the first 100 and then die.
This is what I tend to do, Burn the bend often coming into the home straight alongside athletes way superior to me then die. When I did the 300 yesterday I was very tired and didn’t think I I was ready to run so ended up running a much more controlled bend and seemed to have much more in reserve to finish off. This is something I must take into my races nxt season as I have no intention of running 300m before a race 
Thanks again!