Brave ones, please help me on this one…

If retirement living expenses will be 257,000

If your PTFL will grow at an aftertax rate that of 4.9% (20 yrs), ending with 3,928,000

why do they say that with the ‘usual’ 7% beforetax rate, the PTFL value at retirement (3,928,000) should be adequate to meet ongoing spending needs then ($257,000/$3,928,000 = 6.5% spending rate)
To what are they comparing this 6.5% spending rate? They will need an aftertax rate of 6.5%?
Don’t know what am I missing once again…
— (text CFAI) —
If the portfolio earns total return of 7 percent annually, the value at retirement ($3.93 million) should be adequate to meet ongoing spending needs then ($257,000/$3,928,000 = 6.5 percent spending rate)
(Level III 2012 Volume 6 Portfolio: Execution, Evaluation and Attribution, and Global Investment Performance Standards, 5th Edition. Pearson Learning Solutions p. 107).