The Fibonacci sequence is a simple numerical sequence theoretically used to calculate the length of blocks.
In reality, it is an utter waste of time as it has never once been used properly in the entire history of the known universe.
As a result, there are no real standards for block lengths. Some RW sysops have, in an effort to substitue some other mathematically interesting pattern, blocked for intervals of pi * 10n seconds, with n being fairly arbitrary.
Bizarrely, and for reasons which will never be clear to man or beast, both Edgerunner76 and Linus edited TK's block length in a really futile attempt to maintain the pretense that this blocking system is understood and used. Perhaps they wanted to give some veneer of respectability to the Great TK banning Incident which in every other respect was a prime example of Headless Chicken Mode. Even then, they got the numbers wrong.
Thanks to the vandal bin, blocking is no longer used against vandals, and the sequence has been abandoned. The block length dropdown now contains entries based on the digits of pi. Including, oddly, 0*pi.