Hummingbird Score & Smush

I'm starting to get a mite hacked off with Hummingbird / Smush - I added a new site to my server, configured it and ran an initial test with perfectly respectable 75. I then ran it again having made some refinements, and it dropped to mid-forties. Why? ONE IMAGE that apparently requires optimization! It's not even being used on the site, and WP Smush is configured and has already done it's thing. So I ran it again, this time configuring Smush to include original files. Result? Dropped 3 points! It strikes me as pointless having a barometer of site performance like Hummingbird if it's a) completely inconsistent and b) the score isn't really reflective of ACTUAL performance. Am I just doing this wrong, or does anyone have similar feelings?