MySQL full text search ranking
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  The normalization factor is the middle part of the formula. The idea
  of normalization is: if a document is shorter than average length then
  weight goes up, if it's average length then weight stays the same, if
  it's longer than average length then weight goes down. We're using a
  pivoted unique normalization factor. For the theory and justification,
  see the paper "Pivoted Document Length Normalization" by Amit Singhal
  and Chris Buckley and Mandar Mitra ACM SIGIR'96, 21-29, 1996:
  http://ir.iit.edu/~dagr/cs529/files/handouts/singhal96pivoted.pdf. The
  word "unique" here means that our measure of document length is based
  on the unique terms in the document. We chose 0.0115 as the pivot
  value, it's PIVOT_VAL in the MySQL source code header file
  myisam/ftdefs.h


So, it's a "best practice" pivot value.