Yea it's correct. Americans broke away from Britain and only made a few language adjustments
American English
It's generally a 'Union Flag', but a 'Union Jack' at sea.
Back on topic, American English is the default as that is the most widely used version of English and of course Microsoft are American. It can cause annoyance in England among the patriotic due to American spellings being increasingly adopted as word processors will suggest the American version.
It is still called English when taught in American schools as far as I know, but there are more differences than you might think - see http://en.wikipedia.org/wiki/American_and_British_English_differences;
As to why this happens, well the 'Wikipedia' article can explain it better than I can.
- there are parts of the U.S. that speak somewhat old dialects of English Like where I live currently.
My feeling though is that Americans believe that 'English' is there language because they think they are he biggest power in the world far superior to Britain
If I remember correctly, I believe Churchill said:
"America and England are the only two countries in the world seperated by a common language." 
I don't know about anybody else but I get really annoyed at how programs such as Microsoft Word have 'American English' as a default language. I also do not like to see the American flag next to English when talking about languages. As far as I am aware America speak English as a first language- first spoken in England, so why no Union Jacks??