There's a good chance that volume of exchanges listed on various marketcap sites are indeed fake(especially on Coinmarketcap, where as far as I know, just blindly adds in various exchanges regardless if they're shady). For better and more accurate data on the top exchanges, I'd use this list instead as they have better standards on deciding if an exchange should be listed on their site or not: https://openmarketcap.com/exchanges/ranking/volume