Well I have a couple of ideas, i'm not 100% positive but yeah.
- They are supposed to be a sort of matching pair. Notice how Sawk is a lot less common in White and Throh is a lot less common in Black? They could be drawing similarities to the Gen I Fighting type Pokemon, Hitmonlee and Hitmonchan. Incidentally, all of these Pokemon are male-only. It's not uncommon for Gamefreak to make pairings (Plusle and Minun, Gothitelle and Reuniclus, Sableye and Mawile)
- Their stat spreads. Let's take a look at them.
- Sawk: 75 HP / 125 Atk / 75 Def / 30 SAtk / 75 SDef / 85 Spd
- Throh: 120 HP / 100 Atk / 85 Def / 30 SAtk / 85 SDef / 45 Spd
- Clearly something would be very off if these Pokemon evolved into one another. If like you say, Sawk evolved into Throh, why would it lose Speed and Attack? Surely a Fighting type Pokemon who trains every day would be always trying to get stronger. Likewise, it wouldn't be logical for Throh to evolve into Sawk because losing bulk when you level up goes against the rules of Pokemon :P
That's all I got for now. I'm going to leave you with this picture ;)