It's called the y factor or yorkie factor
they all have it, and I doubt there are many if any yorkies out there that are 100% house trained. Luckily it's just a small mess and if you have the enzyme cleaner on hand it's a quick and easy cleanup.
Don't get frustrated, your pup will probably always have an occasional accident. Just think of them as a forever 3 year old, you know they know where they need to go to, but sometimes they just "forget" or don't make it in time.