Interesting discovery, didn't notice this, must be some logical reason to it.
BMW is sticking with their traditional naming scheme is all. Remember when they had the 320, 328 and 335 all using a derivative of the straight 6....So looking at the brochure the 428d has the 2 litre diesel lump and the 435d has the 3 litre diesel lump I can't find the reason why they are different numbers. The only difference I can see is that the 435 is the 6 cylinder engine whereas the 430 is the 4 cylinder. OK I get that but then the 420 and 428 are the same engine, so anyone care to hypothesise on this?