There is no single price

Open EUR/USD at two brokers at the same second. The prices will not match. On an exchange that would be free money. Here it is just how the market is built. Spot forex is over-the-counter — no central tape. A bank quotes what it will deal at right now; another bank quotes slightly differently because it has a different book to manage. "What is EUR/USD trading at" has as many answers as there are people quoting.