Dear John,
If you look at the table the 2" dimension is the inlet stub length of the Spares Co silencer against 1⅝" for the Armours stainless item.
The inlet stub and and tailpipe inide diameter are both 1⅝" as stated in the last line of the table.
Thank you for pointing out that I had misread the chart. My brain got stuck on inlet and the "stub" part, indicating length, went past me.