Switching Animate Guardian Main Hand Weapon

In my case, I believe I can put a two handed weapon on him first to remove both weapon and shield and start over. But I am asking this question because what happened to me was not really intuitive:

How to switch animate guardian ONE handed main hand weapon?

I have an animated guardian with a weapon and a shield

So first, I put a new weapon and the old one went to his off hand. Ok.
but then when I put a new shield back on him and he kept the OFF hand weapon... like WHY?

I think I understand the coding behind it, but shouldn't it be changed?
I mean if I have a weapon in the main hand, that I just added, and a second weapon that I just forced go to the off hand, it makes sense to me that the off hand weapon should go, no?

please explain to me if my logic is off

thanks!