?? are u referring to the ratio? incl. bay ~=80% for both; ex. bay ~74-76% seems big flag should be higher.
However the actual usable area also depends on wall thickness partition neighbour (say if you are next to power room salute you are so generous and donated a bigger portion to common area...)
Saleable area is the one in 售樓書 which included the windows bay
Net area is what you found in 田土廳 record (or so called 實用面積)
Usable area is net area excluded all wall and column (It is the smallest in value)
Gross area is what we pay based on which is the largest in number.
:em01: