You have to watch out for ads. A M2 is an international measurement, the same all over the world. Good thing you measured. Most people take it at face value (the ad). Many will tell you on the forum that some ads are exaggerated. The only way to get the true measurements is to go to SUNAPR and ask the current owner for the documents/floorplan from SUNARP or like you did measure yourself.
There you will find the true measurements. Be sure to get the original docs not copies. That is why you go to SUNARP with the owner when he pull his docs.
Are you sure it did not have a second floor?
55 M2 is about half what was advertised (LOL).