Mastercard determines a rate (called wholesale rate) and fixes it for the entire day in advance, i.e. the MC rate does not change throughout the day or throughout the weekend. They add a 0.25% markup. They publish said rate at 2pm, depending on where you live. Don’t listen to people claiming you can only see past MC rates.
The interbank rate (and all the plethora of other names it goes by, mid market rate, etc, though there are slight differences between some definitions) is the live exchange market rate, usually taken as the average of the current live sell and buy rates. It fluctuates all the time throughout the day. Because of this, sometimes it does easily swing above the MC rate, and sometimes below the MC rate, which is why you can very well see MC offering a better rate than those offered by Transferwise, Revolut etc, who claim “best rate”, “real exchange rate”, “interbank rate”, “mid-market rate” or whatever the equivalent of the above.
Be careful about claims and hidden markups, for example buy vs sell rate, or weekend rates.
Sell vs Buy: Not all Fintechs offer actual “mid-market” rate. Revolut, for example, has a different rate (or you can say charges a markup) when you convert “A->B” than when you convert “B->A”, i.e. buy rate is different than 1/(sell rate), i.e. you will lose money if you go back and forth at Revolut between two currencies even if the market rate has not actually changed.
Weekends: Exchanges typically close during weekends (i.e. the fx rate will be fixed until monday morning) and some Fintech/banks charge a premium for transactions over the weekend, but may not be upfront about it. Revolut, for example, charges a 0.5% markup over weekend for “major currencies” (vague) and 1.0% for all other currencies … but they’re not very upfront about it, you have to dig into their forums/terms/FAQ to find this out (a google search only finds complaints on their forums, and their search in Help Help Centre | Revolut doesn’t work for me at the moment).
I mentioned Revolut above as that’s the one I’m moving away from … for a plethora of reasons.