Why is 'billion' representing different numbers in Brithish and American English?
It is said words as 'billion, trillion' stand for different numbers in American English and British English ( in old time).

1) Why so?

2) If you are british, do you think 'one billion' as the number 'a one followed by nine zeros' or ' followed by twelve zeros', for example, in public media?

3) From what time the british has changed their ideas and begun to follow the American usage? ---- This question is probably difficult for common people, but I think the shift may have been difficult, because it is likely to create chaos, so I'm curious about it.

Thank you!

Question

Why is 'billion' representing different numbers in Brithish and American English?
It is said words as 'billion, trillion' stand for different numbers in American English and British English ( in old time).

1) Why so?

2) If you are british, do you think 'one billion' as the number 'a one followed by nine zeros' or ' followed by twelve zeros', for example, in public media?

3) From what time the british has changed their ideas and begun to follow the American usage? ---- This question is probably difficult for common people, but I think the shift may have been difficult, because it is likely to create chaos, so I'm curious about it.

Thank you!

Answer

I'm British and I understand that the meaning of the word has changed and I tend to think of a billion as the nine zero version. 
There was no chaos and nobody cares. Language changes and American English influences British English in many ways. 
I imagine that a lot of people don't know the precise definition of a billion and just treat it as a big number that is bigger than a million.

Answer

Over the last fifty or so years the situation has changed. At one time a billion in British English meant 'a million million' or one followed by twelve zeros. Now we almost always follow the American pattern of 'a thousand million' or one followed by nine zeros. This has never been officially decreed and seems to have come about by natural linguistic evolution. Since most people only rarely deal with such large numbers, no significant confusion has occurred.