why was this case different?

Scholars such as Waybray and Hermission do not consider the narrative to be factual, they point out the whole narrative shows traces of wisdom ideology which points to a fairly late date of composition, this is the position I take, although this view is not definitive, see von Rad. The narrative is so well written and engaging that it reads like a fictional story showing plenty of signs of creative flair, it is completely at odds with the cold hearted recital of facts so characteristic of biblical histiography.

There are strong overtones of fate running through the narrative we already know from the text that God and prophets annoint and appoint Kings. As a fictional story one of the main ideas is that self appointed Kings are fated to die in rather humiliating ways. The story of how Absolom ends up being killed is humorous and as I said above has all the traits of a fictional story. From any Wisdom literature you'd expect some sort of hidden take-away as the point of the story since it's intended as allegory.

We have Absalom's whos long hair, his good looks according to the text were legendary and he is David's son -- as you can see elsewhere in the text appearence mattered in the selection of a King and we know he's in good position to suceed David, but then Absalom kills Amnon after Amnon raped his sister. Absalom then flees to Geshur and whilst there starts winning allies for himself by trickery and decit eventually allowing him to travel to Hebron with 200 supporters, he instigates a coup causing David to flee and Absalom appoints himself the new King. David and his forces regroup on the far side of the Jordan and the battle which follows causes 20,000 men to die. Absalom is outnumbered but that's not what causes his death in abttle, no. Instead, Absalom, riding on a mule, gets his hair caught in a tree and gets stuck, while he is stuck in the tree, 10 of the armor bearers of David's general thrust their spears into him -- it's not a very flattering way for a would-be self-appointed King to die in what's portrayed as a legendary battle!