Not the middle exactly, but I'm not surprised to hear your report. It has been several years since I saw any data to convince me that keyword prominence in the title element was still a ranking or relevance factor.
I think that "keyword to the left" has become one of those SEO myths that just lives on and no one tests it. Don't get me wrong here, I'm not saying ignore the advice. I am saying its not a true ranking factor. But, since western languages are read left-to-right, keyword to the left is an eye magnet and does help click-through.
What still matters is single keywords in the title, obviously. But if you're after a phrase and not a single word, and if that phrase is not a strong, stand-alone semantic entity, then proximity of those phrase words is even weak these days.
So now comes the next factor. I wonder if you have some keywords to the right titles you can compare. My sense of it is that you may be seeing a keyword-in-the-middle correlation mostly because that's where your keywords happen to show up.