Every exchange runs its own order book. A price is simply the level at which the most recent trade cleared on that particular venue, so two venues will rarely agree to the cent.
Data providers publish an aggregate, weighting venues by volume and filtering out obvious outliers. Different providers weight differently, which is why two sites can show slightly different figures at the same instant.
Denomination matters too. A pair quoted against a dollar-pegged stablecoin tracks the dollar price closely but is not the same thing, and the gap widens when the peg is under stress.
