Bet it's done in such a way that they can claim "We're just optimizing for Chrome, not slowing down any competitors. It's not our fault our competitors don't using our web engine for their browsers."
I mentioned similar shading behavior on another post, when using Firefox with Chrome or native user agents on the plain old Google search page.
You seem to be quite in the know, would you say that this happens often across the tech space or is this a case of web platform, and in some cases the browser developers acting out?
Oh, I have no idea. It's just something I noticed previously, and has a similar thing to this post, so I thought I'd mention it. I don't have any inside or expert knowledge here.