You're not wrong, but its one thing to top of pressure to spec weekly and losing it. Also important to note that temperature is important when thinking about small variances in measurement.
So while you should check your tires often you also don't really have to fill them often either. A change of 5psi could be an mix of guague error and temperature. Unless your steering feels wonky you really aren't that low.
Tho its still a good habit to top off anyway.