American vs. British "Billion": A Real Historical Numbering Difference
In the American "short scale" system (now the global standard), a billion means one thousand million (10^9); in the traditional British "long scale" system, historically used before 1974, a billion instead meant one million million (10^12), with an intermediate value called a "milliard" used for what Americans call a billion — the United Kingdom officially adopted the short-scale convention for government use in 1974, and short scale is now the standard in virtually all English-language contexts, though the historical difference explains some older British texts and usage.
This is a genuine, documented piece of numbering history, not a myth — the two conventions really did define "billion" differently for a long stretch of time.
Why the two scales diverged in the first place
The short scale (used historically in the US and eventually adopted by the UK) increases each new scale word by a factor of 1,000 (million, billion, trillion at 10^6, 10^9, 10^12); the long scale (used historically in the UK, and still in some other European countries) instead increases by a factor of 1,000,000 at each new word, with intermediate "-illiard" terms filling the gaps — both are internally logical systems, they just define the scale words differently.
Where the long scale is still relevant today
The long scale remains standard in a number of other languages and countries outside English-language short-scale convention, which is worth being aware of specifically when translating a large financial or statistical figure across languages, since a direct word-for-word translation of "billion" can silently misrepresent the actual quantity by a factor of a thousand.