The Colossal Greatsword is uncommon rarity by default. I didn’t take a picture of it in that state but anyone who has obtained one themselves can attest to this. Colossal Cleave, however, is considered a rare skill. And because of this mechanic…
You can increase the Colossal Greatsword’s rarity from uncommon to rare just by disarming it and putting its own skill back on it. You could literally give it back the exact same set of skills it had before, and it would be one rarity higher than it started as.
I personally think this is a hilarious oversight. Non-harmful, but an oversight nonetheless. At this point just make the Colossal Greatsword a rare drop by default, lol. It’s only uncommon to match the rest of Evander’s gear (which you could easily argue should be rare, too).