It's all a matter of controlling where the bias edges end up and having the straight edges on the OUTSIDE of the quilt (or block).
When you cut the setting triangles (aka QST - quarter square triangles), the two BIAS edges (two shorter sides) of each triangle will end up on the INSIDE of your quilt with the one STRAIGHT edge on the OUTSIDE.
When you cut the corner triangles (aka HST - half square triangles), the one BIAS edge (longest side) of each triangle will be on the INSIDE of your quilt with the two STRAIGHT edges on the OUTSIDE.
Even though the cut triangles might be the same size, each is intended for a different position in the quilt (or block).
Now you know.
It's a good thing.
Thank you for the explanation. It's good to know the "why" of things--now I know why and I won't forget "how"!