Interpretating attach graph in --diff mode

[Yuheng Wen]

Hi group! 

I am working with the observational equivalence feature in Tamarin for my protocol. I got an ATTACK but am having trouble understanding why this is shown as an attack based on the attack graph. 

It seems to me that the adversary can simply mirror the current system by choosing the same %uct on the other side? 

Thanks in advance for any clarification!

Yuheng Wen