Not sure what's about Exmo, but they display the best price (by far), only problem is: it seems impossible to get your account verified, tried many times without succeeding.
And the few negative comments I read about it were enough for me to stay away.
So some exchange platforms, when they gained solid reputation and reached a high number of customers, will take a margin on their price, as they also know some traders (or investors) willing to sell ASAP to make a big profit will be fine with sacrificing a bit of their profit if they feel safe with the service. That's probably the case with Bittrex or Coinbase.