It depends if you are sending the emails directly through the stream or are you using a program to send them. They both are calculated differently (When programs are used as content in streams, the engagement score is based on program membership and success status, not email interaction (clicks, opens, unsubscribes). If you are sending them directly you should check with Marketo's support (according to your example).
We don't place much value in the engagement score. Especially since we use child programs in our engagement programs - and track engagement/success at the program level.