I honestly don't know. It is totally counter-productive to handling and ride comfort to fit heavy 20" wheels and then to couple them with even heavier steel uprights which weigh 5Kg more each than the alloy ones. They were specially produced for the 20" cars rather than something which needed using up and are available in both Brembo and HP2 brake fitment.
I can only surmise that something in the load calculations suggested that the alloy upright would not take the stresses of the 20" wheel (or for that matter the 3.3 V8 TDI) and so they designed new uprights from steel instead, and put the larger 92mm bearing in them.
The lower arms are also different for the steel struts - the lower rear has a different inner bush. Possibly stronger?
People have been running 20" wheels with alloy uprights for many years and they are absolutely fine. Whatever calculation it was, there is clearly a substantial safety margin in the alloy stuff. Its probably an extreme use case which no one will ever actually encounter.
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