The bottom case or lid doesn't sit flush

The bottom case or lid (iFixit call it lower case) towards the front of my MacBook Pro doesn't sit completely flush / flat unlike my girlfriends 13" MBP - there's a tiny annoying lip which sits a little high on one side compared to the other when all screws are fully home.

I'm not sure whether my case came like that or whether after a memory upgrade it never sat properly after that.

Is there a particular method for placing the lower lid to ensure a flat and flush fit. I notice on the bottom of the lid there are half inch long nibs to sit into loose black clips and I'm wondering if there is a particular way to sit these?