Okay, let me phrase this a bit better and put the response in terms of the original question and specifically the bit about why some people don't improve much past 59 for the 400 and others get comfortably lower. Lots of people, perhaps a handful of whom are national or world class distance runners, cannot run much under a minute for 400 meters, (You don't like Lindgren as an example then I'll give you Barry Magee.) Some of those people turn out for high school or even college track teams and run 400s. Most of them will probably get below 59 seconds but for some, that's really about as fast as they can go and that's why some of them don't improve more. Others, of course, do not train very well or race as hard as they could.