Instance bBox vs Flattened bBox

Hi,

I want to know the difference between instance bBox and flattened bBox in top cellview.

I think instance bBox and flattened bbox are same In the top cellview. 

However, i saw instance bBox is ((18.185 20.0) (500.0 650.0)). And then i tried to flatten instance cell bBox, and falttened bBox was  ((20.0 20.0) (500.0 650.0)).

why is there a difference between instance bBox vs flattened bBox?

please tell me how bBox is determined.