What exactly is "spendable" income?  Fred's two excellent books provide lots of evidence that you only need about 60% of your working income to maintain the same lifestyle in retirement that you had in your working years.  

So why is 85-115% now considered the sweet spot? 

Does "spendable" income not include costs associated with paying off mortgage or raising children?  

The theory is that once the mortgage is paid off and the kids have left the nest,  your costs decrease significantly so you can retire with 60% of your working income and still maintain the same lifestyle.