An obvious reason is that some of the natural juices of the food seep out and dilute the marinade. A not-so-apparent reason is the effect of the chemical reaction that occurs when alkali and acid are combined in a liquid. More often than not, at least one of the food ingredients you are marinating has an alkaline content that interacts with the acid in the vinegar (or lemon juice or wine), perceptibly diminishing the sharpness of your acidic marinating/flavoring agent.

** Asian Recipes **