I read through a few myself before posting.... I'm guessing it's the Morey calculation Palmer built the spreadsheet around. The three calculation options in the Recipe Builder are pretty close for the beer I'm currently brewing but for some, the three are way off each otehr. The advanced water calculator determines the RA but does not give the target range for the beer being brewed - that's why I use the spreadsheet (hint, developers, perhaps a target RA range and option to use RA to calculate salt and acid effects....).
Thanks for the answer!