IQ is not a precise measurement - especially not at the individual level, and especially not at the highest levels of intelligence when the whole concept of general intelligence breaks-down and there are increasing divergences between specific types of cognitive ability.
There is a tendency to focus upon a person's highest-ever IQ measure - for example in the (excellent!) philosophical novel Zen and the Art of Motorcycle Maintenance the author Robert Pirsig notes the startling fact (and it is a fact) that his (Stanford-Binet) IQ was measured at 170 at the age of nine - which is a level supposedly attained by one in fifty thousand (although such ratios are a result of extrapolation, not measurement).
But an IQ measure in childhood - even on a comprehensive test such as Stanford Binet, is not a measure of adult IQ - except approximately (presumably due to inter-individual differences in the rate of maturation towards mature adulthood).
A document on Pirsig's Wikipedia pages (Talk section) purports to be an official testimonial of Pirsig's IQ measurements from 1961 (when he was about 33 years old) and it reads:
UNIVERSITY OF MINNESOTA
COLLEGE OF EDUCATION
INSTITUTE OF CHILD DEVELOPMENT AND WELFARE
To Whom it May Concern:
Subject: Indices of the Intellectual Capacity of Robert M. Pirsig
Mr. Pirsig was a subject in one of the institute’s longitudinal research projects and was extensively evaluated as a preschool, elementary, secondary, college and adult on various measures of intellectual ability. A summary of these measures is presented below.
Childhood tests: Mr. Pirsig was administered seven individual intelligence tests between the ages of two and ten. He performed consistently at the 99 plus percentile during this period.
His IQ on the Stanford Binet Form M administered in 1938 when he was nine and a half years old was 170, a level reached by about 2 chilldren in 100,000 at that age level.
In 1949 he took the Miller's Analogy at the Univer. of Minn.. His raw score was 83 and his percentile standing for entering graduate students at the University of Minnesota was 96%tile.
In 1961 he was administered a series of adult tests as part of e follow up study of intelligence. The General Aptitude Test Battery of the United States Employment Service was administered with the following results:
General Intelligence .......99 % ile
Verbal Ability .............98 % ile
Numerical Ability ..........96 % ile
Spacial Ability ............99 % ile
John G. Hurst, PhD Assistant Professor
So, as well as the stratospheric IQ 170, there are other measures at more modest levels around 130 plus a bit (top 2 percent).
Of course there may be ceiling effects - some IQ measures don't try to go higher than the top centile.
But still, lacking that age nine test - and most nine year old's don't have a detailed IQ personal evaluation - Pirsig's measured IQ would be quoted at about around one in fifty or one in a hundred - rather than 1: 50,000.
Ultra-high IQ measures must be taken with a pinch of salt; because 1. at the individual level IQ measures are not terribly reliable; 2. high levels of IQ do not reflect general intelligence, but more specialized cognitive ability; and 3. even when honest, the number we hear about may be a one-off, and the highest ever recorded from perhaps multiple attempts at many lengths and types of IQ test.