Information from Sinn, May 2006
All Sinn watches are antimagnetic to 4,800 A/m which is in accordance with the DIN/ISO norm. This antimagnetic rating derives from the high-grade materials of the movement alone. Some watches, like the 656/756/856 aviation watch series and the models 244 and 900, boast a much higher antimagnetic rating of 80,000 A/m which results from a soft-iron inner case.
I was wondering whether the highly amagnetic U-boat steel used in the U-series dive watches affects the antimagnetic rating of these watches at all, i.e. whether these watches have a higher antimagnetic rating than the standard 4,800 A/m. Here is the answer from Sinn (again, summarized and translated inexpertly by myself):
The antimagnetic qualities of the 656 etc. models indicated above derive from the soft-iron inner case which reacts to and deflects magnetic fields. The soft-iron inner case is not inherently amagnetic like the U-boat steel which does not react to magnetic fields at all, much like glass or plastic. Therefore the basic antimagnetic rating of the movement (4,800 A/m) is not increased by the U-boat steel in the U-series watches.
However, amagnetic U-boat steel, because it cannot be magnetically charged, does not affect sensitive navigational instruments, unlike a soft iron case which can carry a magnetic charge (on the outside).
In other words (these are my thoughts): antimagnetic qualities like in the aviation watches prevent the influence of strong magnetic fields on the watch and its accuracy; they protect the watch from outside influences. The amagnetic qualities of the U-series watches work just the other way around, protecting things outside the watch from magnetic interference by the watch itself. This, I might add, is not only relevant for navigational instruments, but also, e.g., for magnetically triggered mines or magnetic detection in general.