Hi

Looking to upgrade my mom's ticket to Rome. She has a premium economy ticket Oct 19 (back later in the month).
I understand that upgrading the ticket would cost the fare difference, and not also a change fee.

A single business class ticket to Rome on Oct 19 is $2100 and change. If I bought two tickets (or more), each is $1500 and a bit. I've seen two tickets cost more when a fare class has but one seat left but this seems unusual. Or is it?