While the long notation or long scale uses multiples of millions to denominate larger numbers (e.g., 10^12 being a billion, a million of millions, thus bi-llion), the short notation (the standard in the US and UK) uses multiples of thousands (e.g., 10^9 being a billion, a thousand millions, which does not match the bi-llion intuition). Most of continental Europe and Latin America uses the long scale whereas most English-speaking countries use the short scale. The difference between the short and long notation is especially bothersome when you are trying to translate between languages that use the long notation and English. Any ideas as to why we have the two scales, why the regional differences and why we ended up with the short scale, which at least to me seems less intuitive?
I can answer the "when" part of the question at the moment; I'm not yet sure of the "why".
From History of Mathematics, Volume 2, by David Eugene Smith (page 85, emphasis mine):
The French use of milliard, for 10^9, or billion as an alternative is relatively late. The word appears at least as early as the beginning of the 16th century as the equivalent of both 10^9 and of 10^12, the latter being the billion of England today. By the 17th century, however, it was used in Holland to mean 10^9, and no doubt it was about this time that the usage began to change in France.
This indicates that the term was first used in Holland in the 1600s. The word was used earlier, though, but only as part of the long scale, to refer to 10^12.