User blog comment:Angel Emfrbl/English Vocaloid + realism/@comment-4042934-20150209033635/@comment-4042934-20150209164614

Exactly. Just like you said, there is variation in the sounds. This is why Vocaloid as is now will only produce a mechanic tone that is semi-realistic,

This can be generalised as there is variation in everything in reality, lighting, sound, pretty much anything. This is the same reason why CGI graphics need a lot of work to match with live action footage.