Rubber seal rear quarter windows

I'm in the process of changing the rubber seals on the rear quarter windows but I've found that the corners aren't sitting flush.

Anybody know how to stop this from happening?

(I've not applied the locking/finishing trim on the inside yet)

I can confirm that after fitting the locking/finishing trim on the inside of the seal it has smoothed out the corners on the outside.

Thanks for updating your post. Being a novice to the maintenance side I use the forum to build my knowledge before I take on a task and it really helps when you do a search, find a thread on what you are trying to do and the person doing the thread takes the time to add how they completed what they were doing