How a Point Spread Gets Its Number
A point spread is a handicap expressed in the units of the sport. Giving six and a half points to one side means the result is settled by the margin of victory rather than by the winner, and the half point exists so that the margin can never land exactly on the number.
Opening numbers come out of the book's own estimate of the game, often circulated to a small audience first so that the sharpest early opinions can be absorbed before the wider public sees a price. From that point the number is managed rather than recalculated. Money arriving on one side moves it; the book is looking for a figure that draws roughly equal interest.
Certain numbers move slowly and reluctantly. In American football, margins of three and seven occur far more often than their neighbours because of the scoring system itself, so a book will usually adjust the price attached to a spread of three before it will move the spread through three.
The distinction between the handicap and the price attached to it is the whole of the craft. A book has two dials and prefers to turn the quieter one.