Why does Jimmy Valentine refer to the young woman as "Polly Simpson"?

Professional safe-cracker Jimmy Valentine has just rocked up at a small town called Elmore. As he's walking down the street to the hotel, he catches sight of a pretty young lady. Immediately, he's smitten. The young lady sees Jimmy looking at her and blushes before walking into the town's bank. After she comes out again, Jimmy starts asking a young boy questions about the lady, as well as about the town of Elmore itself.
He doesn't come right out and ask, "Who is that young lady?" Instead he asks, "Isn't that young lady Polly Simpson?" Jimmy's new in town, and as he knows he's wanted by the police, he's cautious in questioning the young boy. Asking him about a made-up woman called Polly Simpson is a clever trick to try and discover the lady's true identity. It also makes it sound as if Jimmy's more familiar with the town and its people than he actually is. The last thing Jimmy wants to do is to stand out in the boy's mind. That's why he asks the young lad a more indirect question.