Maybe because it is not as important to animate the last part beautifully as the rest of the fighting sequences for the last episode since if the fans can stand some 'quality animation' in the past episodes, 3 minutes of quality dips for the last part in an overall superb episode is a minor thing.
Besides, the background scenery are top notch as always, and the triangler song are good enough to distract me.... I was singging the song at that time instead of noticing some dip in the animation
edit: and they still wrongly put Battle Frontier BFG in those last scene despite it was shown being blown up before, so yeah... it was probably done by different studio just as what adrianop mentioned