Research shows spinal discs have a lot of variation from person to person. In fact it's impossible to come up with one model that represents everyone for scientific study. Your discs and your brother's may be very different.
There could be other damage to the discs from previous injuries that your brother may or may not remember. Age is another factor as well as how much use and stress the discs have taken over the years.
Social scientists claim emotional and psychologic factors are a big part, too. There's really no known single reason for differences after injury between people. It's likely that many things combine together to give us our final outcome in these types of situations.