The 1874 calculation was explained away as being based upon a scribal numerical error in translating 1 Kings 6:1. Therefore the Watchtower Society had a ready made built in scapegoat.
The 1975 calculation was explained away as a few over zealous brothers running ahead of Jehovahs Organization. Therefore the Watchtower Society had a ready made built in scapegoat.
In reality, both calculations are bogus bull$hit because they are based on the assumption that Adam was in the Garden of Eden for 2 years before sin entered into the world.