3 Classics - different output volume levels

I just connected 3 classics to the same amp for the first time and noticed something I did not expect. The output volume is different even when all are set at 50% (the default after reset)

I have a Logitech branded Classic v3 Black Body with Black trim
I have a Slim Devices branded Classic v3 White Body with Silver Trim
I have a Slim Devices branded Classic v3 Black Body with Silver Trim

According to what I have found the internal hardware should be the same across all versions - at least no distinction is made on the wiki page title "hardware versions"

I have swapped amp inputs and cables etc but the Logitech branded one in particular is a lot softer when set at the same output volume.

I am wondering if others have seen this or if there is a different setting I may have missed that would affect the volume. I want to synchronize around the house but in rooms that are sometimes close enough that a difference could be noticed ... so I want them the same.

Thanks!