But what are these differing components then? As far as I see, the turbochargers are also the same:
You can check for yourself and see that the head, the head gasket and both cams are also identical.
To me it is starting to look like after the initial launch of the G20 BMW decided to have just one engine with identical components cover the entire range of trims (318i, 320 and 330i), with the only difference between them being the level of tune. This seemed a bit dull to me at first, but the economies of scale, as well as the opportunity for price discrimination (i.e. being able to sell basically the same product at different prices to different customers depending on their ability to pay) does make great economic sense.
Or do you guys think these databases just can't be trusted?