Fred Franz was not only the "mind" behind the 1975 "Bible-based™" chronology, but was most likely also the genius behind the 1925 "oopsie."
Two things come together to drive this "Bible-based chronology™": the belief in the 7,000 year "creative day" and the belief in the fulfilment of the Israelite Jubilee years, which were 50 years apart. (This may be tied in with the whole dispensationalist* mind-set, but I'm not sure.) That is why there is precisely 50 years between 1925 and 1975.
There have been other minor adjustments in the "Bible-based chronology™," which is why we find close dates like 1874 and 1972 thrown into the mix in some Watchtowr publications. Apparently Jehovah's Magic spirit forgot to tell Fred Franz to double check his arithmetic and also forgotto tell him that there was no year "ZERO."
We are approaching the centennial of 1914, and already some JW apologists have begun specualting that 1914 + 120 years (see Noah) will bring the end in 2034. This doesn't align with the Jubilee model, but Fred is dead, so its time for a new paradigm.
*dis·pen·sa·tion, n.
1. ...
2. ...
3. ...
4. Theol.
a. the divine ordering of the affairs of the world.
b. an appointment, arrangement, or favor, as by God.
c. a divinely appointed order or age: the old Mosaic, or Jewish, dispensation; the new gospel, or Christian, dispensation.