Why does the Watchtower keep changing the rules? Is it Jehovah's will?

"If we were following a man undoubtedly it would be different with us; undoubtedly one human idea would contradict another and that which was light one or two or six years ago would be regarded as darkness now: But with God there is no variableness ... A new view of truth never can contradict a former truth. 'New Light' never extinguishes older 'light,' but adds to it ... The true increase is by adding to, not by substituting one for another."

-- The Watchtower, February, 1881, p3