Unified criteria for water-resistance
To ensure the homogeneity and clarity of the information provided to clients, Patek Philippe has decided to introduce a new unified standard of water-resistance set at 30 meters for all watches certified as water-resistant – having been tested in air and underwater by immersion at an overpressure of 3 bars (corresponding to a depth of 30 m). This measure makes it possible to guarantee the same performance level across all the models concerned and to provide perfectly comprehensible information as to the day-to-day activities in which clients can engage while wearing their watch: washing their hands, showering, bathing, swimming and other aquatic activities, including diving to a depth of 30 m – which corresponds in large measure to actual utilization.
Source:
https://www.rolexforums.com/showthread.php?p=13220522