They obviously need to pad up/alter Arya's story line a bit, as she's done with her ASOS arc, but they can't send her to Braavos too early (will she leave around ep 8 to be shown at the temple in ep. 10, or will she leave in s4 to be shown arriving in s5 only? Opinions are divided over this).

For now it looks like they are finishing to transform her into a "child soldier" out for blood before she's ready for Braavos and the Faceless Men. They started that with a bang, having the inn scene right away because her character development demanded that at this point, right after the Red Wedding.

I think it's not hard for them to invent another fight scene, in which the Hound will get "fatally wounded" at a more critical moment for Arya. In the inn scene, they downplayed Arya's savagery. That was in part because the Tickler had to be replaced with Poliver, but I suspect they kept this in store so a more blood lusty Arya could be shown down the line.

I suspect it's because they intend to structure Arya's story arc and Brienne's to feed one another suspense-wise, giving Brienne some chance, in the non-readers' minds, to actually rescue Arya by the end of the season. First they will have to buy some time with Hound/Arya interactions, as Brienne leaves on her quest for Sansa only in episode 4, based on the synopsis.

Brienne will start by following Sansa's trail but eventually will catch clues about the young girl and the Hound. She will probably miss Arya in Saltpans only by a hair - she could for example find the Hound dead herself instead of being told he is dead - and the Brotherhood, also after Sandor, will fall on her and bring her to Lady Stoneheart. It could mean Arya will first bump into some of the Bloody Mummers again before Brienne does, and it could be how Sandor is fatally wounded. The Hound's death could also happen earlier in the season, leaving Arya on her own, for scenes with Nymeria's dream, and her decision to head for Braavos.