EPC floor area significantly lower than the floor plan provided by agent

The ads shows a floor plan consisting a main building of 126 m2 and an outbuilding of 16 m2 for a toal of 142 m2. However, recently I received my lvl 3 building report and the surveyor, although did not conduct a floor area measurement, wrote in the report that the EPC floor area is only 110 m2. Before this I only knew EPC is for energy rating but not for floor area. So I went on the EPC rating website and indeed it showed 110 m2. I have also checked the houses around the area but their EPC floor area is around 80-90 m2. It makes sense because the seller has extended the 2nd floor into an en-suite main bedroom. However, the discrepancy between 110 m2 and 143 m2 is very huge. What am I missing here? I don't know how these floor area is measured and I am not sure if I should even proceed with the purchase or not anymore. It feels deceiving.