Sorry, the real answer is that a not-too-bright (but very cunning) beaurecrat in the state thought that he could get more revenue for the state by slightly overstating the atmospheric pressure. The difference ends up paying the state 2% more severance tax on every chunk of gas extracted so they can have "lower" incremental tax rates than the surrounding states.
An atmospheric pressure of 15.025 (103.539 kPa) would happen at 500 ft (152 m) below sea level. Atmospheric pressure in the Death Valley isn't that high.
TDK2,
A "micro" definition of standard conditions is really easy, every gas-sales contract has one that both parties have agreed to (it may not be the same as the next contract, but who cares?). The problem is that when you try to define a global standard, people keep bringing their own personal axes to grind. It is never technical, simply political.
David