Hi Tedward1,
Once upon a time, 75% of Sal less the SW deduction was the standard for PHI.
Then in the 1980s, AIDS came along and PHI premiums went crazy whilst insurers tried to understand the risk implications of this syndrome.
In response, many companies reduced the benefits payable for their employees and it's at this time that the twice SW offset became popular. Over time, the risk that AIDS posed became known and was less in general than originally feared. Many companies then reinstated their pre AIDS benefit promise (i.e. 1 x SW) for their employees but some companies kept the 2 x SW in place.