Newton's rings are formed anytime that two surfaces are close to being parallel to each other. There can be many wavelengths difference between the two surfaces but larger distances tend to cause fainter lines as the coherence of the light isn't enough generally to allow for the reliable interference of the light.
The appearance of the rings does tend to indicate that the center area is probably no more than 50 waves or so apart. I don't think that this is how close that the manufacturer really wants the distance to be but if the color response is correctly balanced then I wouldn't worry about it.