The reliability of a bet is calculated with several different variables. Among these variables are for example:
The percentage of the sure bet (unusually high sure bets decrease the reliability)
Last bet date (if the last bet date differs too much, the reliability goes down)
How well the participants’ name match each other
The reliability of a sure bet is meant to be seen as a guideline for you when you bet, but please be thorough and always check the validity of the arb yourself.