fiat money to crypto or crypto to crypto. Each platform do set rates of the currencies and their rates will vary on how sellers and buyers do act.
On the question on why they do have different rate or price rise percentage? It all goes into its own volume of trades. Each exchangers arent correlated. The bigger the exchange
the more price movement you would surely get.
The important thing to look wont really be on percentage rise or drop but rather go directly on the actual price and you might see some arbitrage opportunities.
He is a newbie so I guess he has not seen a lot of these yet in many exchanges. It is not only between Coinbase and BNB but rather across all exchanges. And yes, it is according the exchange's liquidity how this percentage changes. Sometimes the deviation is really wide particularly in small exchange with low trading volume. Anyway, I think the OP will understand it sooner.