Razi's method of simply using the proportion of past successes as an estimate for the probability of future success is not always a reliable way to estimate probability.
Why Razis method not be a good way to estimate the probability?
In the case of your team, they have won 18 of their last 20 matches, so you might think that they have a 90% chance of winning their next match. However, this does not take into account other factors that could affect the outcome of the match, such as:
The strength of the opposition: If your team is playing a much weaker team, then their chances of winning will be higher than 90%. Conversely, if they are playing a much stronger team, then their chances of winning will be lower than 90%.
Home field advantage: If your team is playing at home, then their chances of winning will be higher than they would be if they were playing away.