Standard English is constantly evolving. One of the reasons for this is the great variety of cultures we have in this country. Some from immigrants, some from geographical differences. Another reason English is constantly evolving is technology. As we develop new technology, we are in need of new words. Had you heard of an "app" 20 years ago? Finally, English is evolving because of the media. The media invents new words, but it also proliferates them. The Sarah Palin word-invention discussion comes to mind.
There is no "official" standard English. Standard English is, instead, whatever form of the language is most accepted by the general populace in each country where English is the native language.
Thus, American standard English is somewhat different from British standard English (some spellings, a very few bits of grammar). Overall, standard English is just the kind of English that is seen as "correct" by native speakers and (especially) by the educated classes of native speakers.
A standard dialect, 'has the highest status in a community or nation and is usually based on the speech and writing of educated native speakers of the language'. It is this variety that is taught in schools, described in dictionaries and grammars and taught 'to non native speakers. Standard American English is the standard variety, and British English is the standard British English.