Mirrored plane question....Indirected base available?

I was trying to make a GHscript in order to compute CG and weight of severals blocks (1,2&3).
Initial block CG and weight were include as Key/User values inside the block definition.
Some blocks would then have been moved under rhino (2), others mirrored also under rhino(3).
Each individual CG would then be updated by applying the transformation from the initial block reference plane to base plane to compute the overall weight and CG using elefront or heteroptera plug in. But for the Mirrored block i was expecting a (X,-Y,Z) base plane instead of a (X,-Y,-Z) which would have give us a block in the green position…
So is there a limitation to indirect plane??? could’nt we get the complete transformation matrix beetween two blocks??