Why can the land area in hectares shown by the Reviiri service sometimes differ from the area reported by Property Services for the same land plot?

One reason is the coordinate system used for measurement. Reviiri measures the area by taking the corner points of the land plot as defined in the KTJ in the ETRS-TM35FIN coordinate system and then calculating the geometric area based on those points.

Another reason is the time since the area was measured. If a long time has passed since the measurement was carried out, older measurement methods were not as accurate as those used today, and even then the area differences have usually been only around 1% or slightly more.