It is often a situation of condition, as any drain on weight will essentially leave a body weaker then when it is running optimally. We tend to look at the chin, and not the other many determining factors, conditioning, age, and as Andre has suggested reoccurance and it's effects, as Missy suggests, mental toughness. etc.

While fighters often either do, or don't have a chin, this being established usually early in their career, there are still many things they can do physically to increase it's durability. So therefore also, when a fighters 'loses their chin' again, many variable can also come into play.