Several reasons:
1) Because a lot of people, if Oly has the same line as is available elsewhere, and you already called, will just bet it anyway. Some won't because they are pissed, but a lot will.
2) It's been rumored (ahem) that Olympic uses Don Best as a method for betting into shops which follow them. Spiro likes one side of a game. He posts a line move on Don Best while not actually moving his number. Others blindly follow him. He bets the side he likes at a better number.
This is why I have said for years (ever since I became aware of how it actually works) that the Don Best software should be tied into the wagering software of the book. No human intervention needed (except when there are problems) and the line would reflect what is actually available from the book.
Good Luck,
Bart