I am not very techy
OK, so I won't go into the "why's"
Your attenuation depends mainly on two things- the length of your line (the longer the higher) and to a lesser extent your sync speed (the higher the higher).
It's also not a particularly accurate figure, so a variation of 1db is neither here nor there (it's about 25% in power terms, 12% in voltage).
And then there are rounding errors... 55.4db would be reported as 55db, 55.6db would be reported as 56db.
Believe me, it ain't worth worrying about
It's also nothing to do with IDNet, it's purely down to the physics of that copper wire between you and whatever piece of kit is on the other end!