Rounded corners with Kivy (using Python only)

I have this little Kivy app (Python version: 3.7, Kivy version: 1.11.1):

Code#1

from kivy.app import App
from kivy.lang import Builder
from kivy.config import Config
from kivy.uix.floatlayout import FloatLayout

Config.set("graphics", "width", "500")
Config.set("graphics", "height", "300")

kv = """
<RoundedCornerLayout@FloatLayout>:
    background_color: 0,0,0,0
    canvas.before:
        Color:
            rgba: (.4,.4,.4,1)
        RoundedRectangle:
            pos: self.pos
            size: self.size
            radius: [(40, 40), (40, 40), (20, 20), (20, 20)]
"""

Builder.load_string(kv)


class RoundedCornerLayout(FloatLayout):
    def __init__(self):
        super().__init__()
        self.size_hint = (None, None)
        self.size = (400, 200)
        self.pos_hint = {"center_x": 0.5, "center_y": 0.5}


class MainApp(App):
    def build(self):
        return RoundedCornerLayout()


if __name__ == "__main__":
    MainApp().run()

And with that code, I have the following result:

Cute, isn't it?

Now, let's try to get the same result using only Python. I'm trying with the following code:

Code#2

from kivy.app import App
from kivy.config import Config
from kivy.graphics import Color
from kivy.graphics import Rectangle
from kivy.uix.floatlayout import FloatLayout

Config.set("graphics", "width", "500")
Config.set("graphics", "height", "300")


class RoundedCornerLayout(FloatLayout):
    def __init__(self):
        super().__init__()
        self.size_hint = (None, None)
        self.size = (400, 200)
        self.pos_hint = {"center_x": 0.5, "center_y": 0.5}

        self.background_color = (0, 0, 0, 0)
        self.canvas.before.add(Color(.4, .4, .4, 1))
        self.canvas.before.add(Rectangle(
            pos=self.pos,
            size=self.size,
            radius=[(40, 40), (40, 40), (20, 20), (20, 20)]))


class MainApp(App):
    def build(self):
        return RoundedCornerLayout()


if __name__ == "__main__":
    MainApp().run()

Fair enough, I thought.

But then I got this result:

As far as my knowledge goes, both instructions (Code#1 and Code#2) are saying the same, but in different ways. Scientifically proven, it's not like that.

...So what I'm trying to understand here, and the point of my question, is: what's the functional difference between Code#1 and Code#2? Why are they displaying different results? And what would be the correct way to "translate" Code#1 into Python-only code?

Ignore the fact that just keeping the kivy code is the easiest solution. What I need here is to understand this behaviour, explaining my reasons would unnecessarily extend this question, let's just say you can only control what you understand.