I apologize, I didn't see the possibility to attach a picture. I'll show you an example.
On the left is drawn an ellipse of white color with alpha channel = 70 and the layer is set to transparency 255.
On the right, a white ellipse is drawn with alpha channel = 255 and the layer is set to transparency 70.
See what I'm talking about? It looks like the right ellipse has antialiasing applied, but the left ellipse does not.