My Father was diagnosed with this cancer and unfortunately recently passed away as a result of it.
The doctors treating him never specifically called it a "pancreatic cancer" though they did say it was closely related or "on the fence". The ampulla's close anatomical proximity to the pancreas and the fact the pancreatic juices pass through it probably explains why they would refer to it in this manner. Just as a guess, I suspect the lack of specific relearch with this cancer may also lump it in the pancreatic box due to the lack of treatments which specifically target it.
Regards
Martin