Assume we have object A(mass ma), liquid and container as B(mass mb)

If we just put the liquid and the container on top of the measurement device, due to gravity g
B will have  mb*g download force acting on the device, that is how the device get the reading mb*g.

If we put object A into the container, the total force acting download will be ma*g + mb*g

If the object is floating on the liquid, the weight of the object is support by the liquid. i.e. the liquid provide upward force on the object (ma*g up) to support the gravitation force for object (ma*g down).
So the same time, the object give the liquid ma*g downward force. Now the total force acting on the liquid is ma*g + mb*g (extern force from object and gravitation force due to it's own mass). So the measurement device need to provide ma*g+mb*g upward force to support the liquid. That is why the reading will be ma*g+mb*g