[QuickSilver]

I'm not sure why it is, but I think part of the problem is that they are so small and fast. I've posted before that Thor can squirt out a pee in under half a second. He does it all the time in the park while he's walking around. His back legs go out a little wider than usual for one stide and he's done. Literally, if you blink, you will miss it.

I went for a period of time thinking that Thor was house trained when he was not (actually, this appears to be a continuing pattern, but he is much, much better). So I think sometimes people may miss accidents and think they are 100% consistent, while their yorkie still thinks it's okay to pee inside once in a while. Just a theory!