It doesn't look like you are too far off. You are within the range I'd expect to see from a rounding error somewhere. I'm not sure what calculations the admin dashboard uses, but there are a lot of moving parts to calculating size.

Just for fun, you might try adding up the size of all of the pages yourself to see if what you come up with. Each page has img_size, lft_size, loc_size, and txt_size.