It's most likely from a strong culture of interaction design and usability testing. During the design process, you usually do usability tests with would-be users and they can help illuminate issues with your design. In this case, it may have even been during testing that a person said, "well that's not what I meant by tomorrow."
Pretty much every time I do a usability test or study, random users catch small things like this that help make a product much better. When you design without feedback, you have your own mental model of how everything is supposed to work. But you're the designer and the expert, and users are not. They often find different ways to use your product than you intended.
But it's even more impressive because you could do usability testing all day long (during work hours) and never catch this issue. It's when someone's _really_ using it where something like this comes up.