This may sounds like a reasonable wording, but is very often not the case unfortunately. I believe it’s because you’re salaried rather than paid by the hour. Even though your contract is for a set number of hours, you’re expected to have just one job, and for that job to get your focus and attention in a way that you wouldn’t be expected to do if you were paid hourly. This means that companies often technically own the IP you develop in your own time, and big ones will enforce that.