According to their site:
"Red Boat uses only the freshest cá com (black anchovy), salted minutes after leaving the sea then slow aged for over a year in traditional wooden barrels."
Could it be the aging process continues once the bottle is open so storing it in the fridge keeps it at its optimal flavor?
Like Bourbons and Whiskeys once the fish sauce is removed form the wooden barrels they are aged in the maturation stops. According to what I have read Red Boat only recently added the "refrigerate after opening" to their label and that not many people refrigerate theirs. They say that you risk precipitating the salt out of the sauce's solution if it is chilled however in high quality sauces like Red Boat it isn't really a problem. Red Boat does say best used with in 2 years of the manufacturer date.