You check the rate online, then your bank quotes you something noticeably worse. You have not been singled out — almost every provider does this. The reasons fall into a few predictable buckets, and once you know them you can shop around effectively.
The biggest reason is the mid-market rate versus the retail rate. Banks take the wholesale mid-market rate and shift it in their favour before quoting you. This markup is invisible because it is built into the exchange rate itself rather than shown as a line-item charge. A provider can truthfully advertise "no commission" while still making a healthy margin purely on a wide spread.
On top of the spread, some providers add a flat fee or a percentage charge, especially on international transfers or non-sterling card transactions. Always look at the total cost — the amount that leaves your account versus the amount that arrives — not just the headline rate or the fee in isolation.
Abroad, when a card machine or website asks whether you want to pay in your home currency or the local one, always choose the local currency. Paying in your home currency hands the conversion to the merchant's payment processor — a practice called dynamic currency conversion (DCC) — which almost always uses a poor rate with a large markup. Choosing local currency lets your own card network do the conversion, usually much closer to mid-market.
The wholesale market is thin or closed at weekends, so providers widen their spreads to protect themselves against rates moving before they can trade. Exchanging large amounts on a Saturday can cost more than the same transaction on a weekday.
Pick the live mid-market rate as your benchmark, then for each provider work out the rate they are actually giving you including every fee. The percentage gap from mid-market is the real price. Our converter shows the mid-market figure so you always have the benchmark to hand.