Let's take a pound of fat as 3500 calories. To gain one pound in a year is an average of 9.59 excess calories per day. Or about 0.5% of the typical total daily intake.

Yeah, managing a system within 0.5% is subtle.

Especially when biologically and psychologically the pressure is towards over consuming rather than under. If you consistently eat a deficit you will very obviously feel hungry. If you consistently eat a small excess the effects that would lead you to regulate are much more... subtle.