In my experience, the two most big factors of "Unknown" are.
1- Non browser traffic. Bots, search engines, malware attacks, etc etc.
There are LOTS of non-browser traffic. The less visited is a web, the more non-browser traffic it has.
2- Android/Android derived OS
Android derived has too many versions, and lots of browsers and apps with web traffic. A lot of these traffic uses weird/unrecognisable User-Agents so in classifications like this they end as "Unknown".
3- Apps in general. Obscure browsers. Privacy add-ons. Rare configurations
Far behind the previous classifications, you will find traffic from these other cases.
For this, I would just split half to Android, and the other half I would just remove it and weight it between all the remaining shares (because most is just the first option).