Why does a verbose person talk "nineteen to the dozen"?

Though it is by no means common, I've heard this expression multiple times recently, and I'm wondering why it isn't "eighteen" to the dozen, or "thirteen", or "twenty". Where did "nineteen" come from?

I can't vouch for its veracity, but here's a site claiming the expression has its origins in the Cornish tin mining industry.

Beam engines were introduced to reduce flooding in the mines, and they pumped out 19,000 gallons of water for every 12 bushels of coal needed to operate the engines – a much faster and more efficient way of pumping water than the hand pumps they replaced.