Offshore is 100% spot on and so are the others who mentioned that Atm are non attributable directly to depth.
ATM is a measurement of pressure not depth and pressure is dependant upon the medium and other environmental factors, such as altitude, and global position (lat/lon).
All of these factors and more contribute to the pressures felt and therefor NO device or table is ever 100% accurate, hence the fact that US and UK and other diving tables give a conservative estimate based on the law of averages and statistical possibilities, they are not factual but merely a guide, as to the best estimates availabe and also conservative to try and be as safe as possible.
Edge!
Chemist/Physicist