Doubt in choose a new agent

Hello experts,

I have a little doubt when I choose a new agent in a work item.

Example:

Go to SWIA and choose a new agent:

Before this change, the work item had these agents (7 agents):

But, If I choose only ONE new agent, the work item will be send to the new agent and more other two agents:

My doubt is: Why it happens?

Thanks in advance!