Why did the roman army become so "strange" later o

[markusaurelius]

I would look at Trajans column as a "snapshot" of that point in time. Perhaps during that time or the specific legion that was used by the artist to make his plan wore lots of segmentata. It would be the same if a hisortian 1000 years from now looked at the surviving photo album of a WWII American soldier. They would paint that brush across the whole century baring any other evidence. Trajans column is exactly that in my mind.