They look different because they used two different forgings. The one on the left is the older style. They started using the newer style forgings around 10 or 15 years ago (IIRC).
Look at the width (front to rear) of the reinforced area around the receiver extension hole (where the extension threads into the receiver). You'll notice that the reinforcement is wider on the new style receiver. This is not machined; it's how the receiver came out of the forging die.
They started doing this to reinforce the receiver at the point where the extension screws in.
Both receivers are correct. I have Colt receivers of both types, bought from Brownells and elsewhere.
ETA: The flat on top is machined. If you measure from the top of the flat to the top of the main part of the receiver, the measurements should be the same for both. I also wanted to mention that Colt apparently still has plenty of the old forgings, since they seem to still be shipping them.