Help needed on a scientific anomaly

I pulled out a grey hair just now - I know, better stop doing that. It was a crinkly one that stuck right up. On closer investigation, this is what it looks like. The end at the root is dark, my normal colour, and smooth- that is, not crinkly. Half way along the hair, it turns grey, and goes crinkly.

I thought when your hair turned grey, you stop growing brown ones and start growing white ones - so I don't understand how a half and half hair grows. This is the second one I've noticed like this. I've kept it!