The stack doesn’t quite work like that. They would get added like this:

This > buffer
great, is, one > stack

Then, each + pulls the latest off the stack and operates.

First + would pull ‘one’ as it’s the most recent on the stack.

This page does a good job of explaining: https://www.geeksforgeeks.org/dsa/introduction-to-stack-data-structure-and-algorithm-tutorials/

Edit: you can also see this reflected in the Alt-text:

I NOTATION POLISH REVERSE ❤️