I have recently aggregated 6 spatially identical MapInfo TABs in FME. The data has Lotplan and PPN attributes that are then used to group by in the aggregation:
The polygons and figures in the resultant output appear completely fine with all the Floor Area (float)attribute data being correct. Then these polygons are taken into MapInfo and split by another polygon layer with Floor Area figures split proportionally based on the new split area. The problem is that the resulting Floor Area data is divided by 6 (the number of input datasets) consistently across the dataset.
I have worked around this problem by multiplying the figures by 6.