How can builders advertise houses as bigger than they are?

I have been checking the sizes (Length by width) of plans from several builders, only to find that my calculations (LxW) are often less than what they advertise the house as. For example: A 12220 by 8050 house by my calculations is 9.83 sq house, but it is advertised as 10.5 square. (And the verandah is already included) 

The calculations are done from their plans, from outer brick to outer brick, so there is absolutely nothing else to be added into the calculation, unless it's the eaves?? Would eaves be part of the sq metreage, but not on the plan?? 

If not, is there any other reason they are allowed to advertise it as larger than it actually is?? 

This is also a warning to buyers to double check what you are REALLY getting!