Freddy Franz working as the main writer/oracle under the Knorr administration, came up with these very magic numbers.
He said that Mankind would be 6000 years old in 1975, despite all scientific evidence to the contrary. More importantly- Somehow, he made people believe that 6000 was a magical number, necessitating the destruction of more than 99% of mankind that year even without a single scripture in his new Bible (which he wrote).
Oh, you may say there's scriptures that say 1000 years is as a day to God, but we aren't talking about a day or 1000 years. We are talking about 6000 being six-sevenths of the single day of rest, and the millenium rule of Christ being the last seventh, creating the other magical number- 7000. Not a week, but a single day of God's. Without a single scripture, Franz convinced perhaps millions of people that God created everything in 6 periods of 7000 years described as the "days" of creation in the book of Genesis. He convinced them that all life on earth was less than 48,000 years old.