I asked around. No one knows for certain why it was made the way it was made. The folks who worked on it are long gone from Roblox.
Just so we understand correctly. You’re referring to the inside of the arms and legs, right? So in the third image, the purple part? If so, I think the original creators thought that part was never meant to be see when the arms / legs bend, and so just didn’t spend time making it seamless.
Since this is non-critical, we’re not going to make any updates at this time. As you can imagine, making updates to the default blocky would have a mass effect on millions of users.
Would it be possibly to ask one of the former/retired admins from way back when this was made possibly just to have a clear view on what it’s supposed to be, or not possible? Even if there’s no way to fix it either it has to be re-done in 2025 from 2016, or just leave it untouched for the rest of its days. Still unfortunate to leave it how it is though.