Difference between measured perimeter and calculated perimeter

Hi all

I am trying to cauculate the perimeter of the geometery in my model.

Firstly, I used the measure function to acquire the perimeter.

Secondly, I defined a interpolation function intop(1) on the boundary of the geometry and I tried to compare them.

However, what I found is that there is difference between measured perimeter and calculated perimeter. And calculated one is smaller than the measured one as well as the real perimeter.

Is this allowable discrepency or the error of the defined perimeter-calculation-function?

Thanks in advance.

Best