You still don't understand this? The long term DTD injuries are modeling chronic/overuse injures (ie niggles) rather than acute injuries.
Something like a stress fracture, inflamed joint, or tennis elbow is perfectly modeled with long term DTD injuries. You can rest it for several weeks, but you'll still irritate it again if you go back to playing the sport. You can either play through it (with associated drop in performance and risk of further injury) or you can rest for a long time and hope the problem goes away (but it often gets re-aggravated as soon as you start back up). Chronic injuries like this don't have some set time line like acute injuries often do, they flare up over and over again due to aggravation from playing. A 3 month DTD injury is the perfect approximation of a chronic injury.
I think Peyton Manning was listed as DTD for the last 3 fucking seasons for the Broncos