And that index is not even accurate now. The rates are very different from banks to bank right now.
Here in Russia you can exchange through various banks and every bank has different exchange values. Sometimes considerable differences among them right now.
For example, SberBank (which is not even the best for the rates of exchange), now
let you buy 1 USD for 59,75 Rubl.
and sell 1 USD for 51,43 Rubl.
This large spread between buy and sell is due to Russian decisions to protect the ruble since the first sanctions.
Since there is this large spread, also differences between bank rates can be also huge. Indeed, this large spread will make sure that these big differences between banks do not create any arbitrage opportunities.
Rosbank, on the other side,
is something like sell 1 usd for 45 rubl
and buy 1 usd for 61 rubl at the time of writing.
And still no arbitrage opportunity comes in.