The conversion tool on this site multiplies an amount by a number published by a central bank. That sentence is the whole of what it does, and the distance between it and "what will my bank give me" is worth spelling out.
What a reference rate is for
A reference rate is fixed once per publication day by a defined procedure and published as one number per currency. Its purpose is to give everyone — companies preparing accounts, statisticians, contract drafters, tax authorities — a single agreed figure for a given day.
That is a real and useful job. It is not the job of telling you the price of a transaction.
What it is not
A market rate is a quote from somebody willing to trade. It has two sides, moves continuously through the day, depends on the size of the transaction and on who you are, and includes whatever margin the provider charges.
A reference rate has none of that. There is no spread in it, no fee, and no counterparty obliged to deal at it. The gap between a reference conversion and the amount a bank actually credits is not an error in either figure — they are answers to different questions.
The publication calendar is part of the data
Reference rates exist only on the publisher's working days. There is no rate for a Saturday, and none for a public holiday of the publishing institution.
The honest response to a request for a rate on such a day is that no rate was published. Substituting the previous business day's figure produces a number that looks like data and corresponds to no publication, and it silently misdates every result built on it. Our converter refuses instead, and says which currency and which date it could not find.
Cross rates are arithmetic on two publications
The publisher quotes every currency against one base. Any other pair — dollars to pounds, for instance — is obtained by dividing two of those quotes.
Both quotes must come from the same publication day. Dividing one day's rate by another's produces a figure corresponding to no moment in time. The converter uses two quotes from a single date and reports that date with the result.
What the tool tells you
The result comes with the amount, both currency codes, the rate used, the publication date it came from, and a sentence saying that this is arithmetic on a published benchmark rather than a price you can trade at. That sentence is not decoration; without it the number reads as a quote.