It's good to wonder why things are the way they are, as a programmer, but when it comes to "why does someone else's code work this way?" questions (including ones about the standard library), in general these aren't a good fit for a Q&A site because they don't offer insightful answers. Generally, the author of the other code simply had to make a choice, and only the author knows the reason for that choice - others can only speculate.

I think there's some reasonable speculation to do here, so I'll offer my own answer. But for the future, consider restricting "why" questions to the cases where you can't identify the underlying logic: i.e., you've reduced a problem to a simple example, and you get a result that isn't what you want, and you can't identify the pattern behind that result.

You'll notice that Michael's answer basically substituted a "how" question: "how do I get a substring match instead?".