[Peck]

If u want to get really technical:

The gravitational attractions the robot causes and any magnetic forces provided by the robots near location might be causing an upword acceleration to the robot thereby holding weight.

Of course, The same argument could be used to say that the earths magnetic field and the gravitational attractions of the roof, and everything above it as well as the magnetic attractions caused by the lights and the wires...

Might I recommend not trying to get down microscopic in your arguments so this all doesn't come into play since the roof could very well be lifting as much weight as any robot.