Panopticlick works by comparing your browser's fingerprint with what it has seen so far. So when you're the first or one of the first users that access it with a new version, it hasn't recorded data from other Tor Browser users yet. After a while, more people visit it using Tor Browser and you're not unique anymore.
I just visited panopticlick with the newest version of Tor browser.
The property with the most bits of entropy by far was "Screen Size and Color Depth".
Tor Browser is strange in that this property is very variable; if you resize the browser window or go to full screen mode the value changes. This is not the case with other browsers such as Firefox, where "Screen Size and Color Depth" reflects the resolution of the monitor that the browser is displayed upon.
I don't know the reasoning behind Tor browser doing this differently, but it it probably the source of your fingerprint's uniqueness.