Effectively each manufacturer will design their GPU slightly differently, and may apply different configurations to it.
So, one may have:
-More effective cooling
- A high core clock speed / turbo speed
- High RAM speeds etc
- It may come already overclocked
and so on.
Equally, in certain markets, some manufacturers may only have a limited supply of their GPU, in which case, their price will rise as demand is still high. I wouldn't expect to be this case in this situation, but it occurs none the less.
What are the exact models of either GPU you are looking at?