The final battle with the dragon is different for several reasons. First, the motivations for fighting the dragon are different from that fo Grendel and his mother. The original reason for Beowulf to fight Grendel and mother was to provide service to Hrothgar, because Grendel kept attacking his people and being a nuisance.
Beowulf fights the dragon for different reasons. Instead of doing it as a service for someone else or to protect people from danger, Beowulf elects to battle the dragon for selfish reasons, telling his men:
"This is not your fight, nor is it fitting for any but me alone to test my might against this monster here and achieve heroism. I shall win that wealth mightily." (XXXV)
Another factor that influences Beowulf's defeat that did not occur in the fight with Grendel and his mother is the abandonment of Beowulf by his men. All "ran off to the woods to save their lives..but one" (XXXV) When he fights Grendel's mother in the lake, Beowulf is alone, but his men stayed on the shore and "stared at the waves; sick at heart, they wished and yet did not expect to see their winsome lord again" (XXIII)
Moreover, Beowulf's noble sword, Naegling, "was splintered; Beowulf's blade, though ancient and gray, was broken in battle" (XXXVI). The dragon's fire had already burned the shield "down to the boss," so when the dragon assaults Beowulf again, he is without a weapon to defend himself.