I have parcel data from Landata, the agency that looks after land parcels in Victoria, Australia. I am finding a huge difference in the area calculated in QGIS for the parcel compared to onground surveyed results. For example, a site that is 12.32ha is around 16ha. I'm using a VicGrid projection ESRI:102171 and the data is in GDA 94, EPSG:4283. Would this cause such inaccuracy or is it more likely the data itself is just a representation of the parcel given the dataset covers the entire state?