While it has vastly improved in terms of accuracy, its far from perfect.

Though there is no official way to retrain Siri using your iPhone, there are some viable ways.

It is just to enhance Siris capability to analyze the voice accurately and react to it promptly.

Tap on Siri Search

Give these handy tips a try and Im sure they will work wonderfully.

Summing Up..!

Disable Siri on your iPhone

Train Siri to Recognize only your voice using iPhone