They are typically not 100% but really close. The shape of the parcel is normally correct, and you can use it to figure things out. The data they use comes from your state or county GIS parcels, which is public information. For a single piece of property the surveyed plat is the most accurate and legal boundary. However, the GIS parcels are basically all plats and how they tie together but the lines can get slightly generalized or off. Here is an example from one of the public lands I hunt:
The yellow line is the GIS parcel the blue line is how the line is marked on the ground by the public game lands (following the logging road on the ridge). I would say that this is about avg accuracy for GIS on large parcels, some are more some spots less. The plat says the ridge-line is the property boundary the surveys then likely shot the highest, furthest points they could see in one setup because time is money. So in this case the parcel layer might be spot on what was surveyed or not, and the logging road does follow the ridge-line nearly perfectly but not everywhere.
There are several layers that could make "off" the GIS parcel layer, the aerial layer, the person on the ground marking it. All are trying to be as accurate as possible but if all three are off in different directions it compounds the issue when you put them all together in one image but normally they fall close, but again the plat and survey stakes are the real line, except when the plat says the line is a topological feature...like a ridge or river which can shift.
Hope that helps, and is as clear as mud. I tried to keep it simple without going into the inherent problems of mapping a round earth on a flat piece of paper. And the problems with private people pushing out their boundary line with signs, paint, fences a little every few years hoping no one notices.