Just curious why ... No hardship endured.

AA 2167 is STL-DFW-SJO. I fly the DFW-SJO segment regularly, but yesterday did the whole thing for the first time. First segment used a gate in DFW A terminal. I got off for the hour-ish break with all my things. Made my way to the designated D gate to continue on. It was still a 757 but a different plane. (Several little things I noticed that were different.)

For all I know, maybe most of the time it is the same plane, and just wasn't yesterday.

My question: What is the rationale for designating this as a through flight? Is it to create an impression that STL has flights to Costa Rica? The monitor at the STL gate flashed back and forth "Dallas-Fort Worth" then "San Jose, C.R."

When I selected seats at aa.com, it showed several seats as open, but only one seat (3F) that was available for both segments, and that was the only one it would let me choose.

All fine, but it just seemed odd.