It is one of the quirks of AMG ownership. Many believe (wrongly) that almost every part will be stamped or labelled 'AMG'. I have lost count of the amount people who have proudly proclaimed to me "that's not a real AMG, it doesn't have xxx".
What I have learned over the years is that AMG (like many other tuning houses) used what was available to them at that point in time. They also (in the earlier days) concentrated on function, over styling, for many parts of the car. My guess would be, On the 55's they used the bigger front calipers, as they were functional requirements and the 'stock' Mercedes calipers on the rears, as they were functional and available.
IMO; The car would suit the bigger 'AMG branded' rear calipers. But then you lose the originality of the car. That said, as AMG were building around a stock Mercedes shell, you could argue that is exactly what they were doing ;^)
One of the other "quirks" in the 55 is the front end of the car is 'stretched' to accommodate the V8 and transmission. This meant using longer wings from the stock W203. All the arches are rolled to allow for the wider and bigger wheels. There are so many small differences that are unseen. People always focus on what they can see and what they believe is 'correct'.
