1) The background of the speaker can effect pronunciation and also lexicon.
2) Standard English, or any standard language is one in which the norms of usage have been agreed on and have been codified, eg in dictionaries. Countries may have different varieties of English, for example, American English may have slightly different spelling in some words than Australian English. Words can also differ in meaning, as well as pronunciation between varieties.
3) Brittish colonisation was largely responsible for the spread of English in the early days, nowdays its spread can be attributed to the economic dominance of the US. Its also the main lingua franca of buisness, technology and academics, which helps its growth.