That's because the law of one price is purely theoretical and never applies in real life https://en.wikipedia.org/wiki/Law_of_one_price
There is no such thing as a "live rate" or "market price"
There is no such thing as a "live rate" or "market price"
In other assets, like equities, there often is a single central exchange. If you want to buy CURI for example, it's only listed on NASDAQ, so the price on NASDAQ is The One True Price as far as a normal person is concerned.
There's no such central exchange for currencies.
In fx it’s the law of whoever has the most information because there is no benchmark market at all. It’s why fx pricing is so hard to do right.
If you talk to 3rd party institutional FX providers, they'll immediately mark up rates because its how they make all of their money.
All currency exchanges track each other extremely closely, because it's extremely easy to arbitrage between them.
Forex is essentially the largest, most liquid market there is.