By definition, when tracking a record with a pivoted tonearm, the cantilever is aligned perfectly perpendicular to the groove in only two instances.
The longer the tonearm, the smaller the offset error will be between these two instances, resulting in lower distortion.
This explains why LPs tracked with the 12.1-inch long tonearm sound just a tad more poised, relaxed and natural.
By the way: The 12.1 is not just a 10.5 with a longer armtube.
Since a longer arm tube offers lower torsional stability and is more prone to resonances, they had to implement quite a few structural changes.
Brinkmann Tonearm 10.5 & 12.1, are proud of the fact that you will probably not notice any them at first sight, and even more proud of the fact that the 12.1 sounds even better than the 10.5