I don't know about Siri specifically, but it is based on Nuance software. Dragon Dictate attunes to your voice and vocabulary over time IF you take the time to correct it by voice. Each time you correct it to a word it didn't know, it adds that word to its vocab, and if it simply mistakes the word, then after three or four corrections it learns that the corrected word is a more likely match for that particular sound.
Apple says that Siri gets better over time, so it must do something broadly similar.