Could be, I don't know. I doubt that video creators would go through all of these old videos just to update them to a slightly higher bitrate; the other possibility is that YouTube kept the original uploads or higher bitrate variants without previously showing them (and only showed them now), but that seems like a huge waste of storage, so it seems unlikely to me. Again, we're talking about old uploads (2-3 or up to 10 years ago), not new ones.

The one thing I've seen that makes sense is updating old videos that were previously available at up to 480p and bringing them up to 720p or 1080p (with the idea of keeping the original published video with the views, comments and so on instead of uploading a new one).