Multipath errors result from signals that are reflected off objects near an antenna. These reflected signals have associated time delays, since they travel an extra distance. Even a few milliseconds of travel time delay can wreck precision. These errors tend to average out over longer periods of time as the satellite makes it traverse across the sky. Obviously, the longer the sampling time, the more likely these errors will cancel each other.
Human error is also a major factor in periodic surveys.
Continuous GPS sites have fixed antennae, so the errors associated with centering the GPS antennae over monuments are eliminated. Every millimeter that the antenna is misaligned over a monument will result in a millimeter position error. Additionally, failure to orient the antenna to north and level the antenna will add even more error. The stability of the tripod itself also affects precision. The ambiguity resolution is another problem that is greater for shorter sampling periods.