Variance in Internal Size – when buying off the plan

We bought off the plan 12 months ago & have now been in to look at the very nearly finished product.

When we bought, we were told the apartment was 72 square metres internally with a 17 square metre courtyard. The registered strata plan says the total square metres internal & external is 91 square metres, so we have gained 2 square metres somewhere.

However, when we measure the individual rooms of the apartment the total internal floor space is 63 square metres – not 72 square metres.

Is there a ‘trick’ to how they determine the internal floor space – because we seem to be out by about 9 square metres.

Thanks