IQ scores - fixed raw scores vs infinite tails of the underlying normal distribution of standard scores
The answer is provided by the Test theory, which distinguishes between the true value of a latent (=unobservable) psychological construct, in this case the IQ of a person, and the IQ score measured with an IQ test. A good test is objective, reliable and valid which ultimately means that each measured score reflects the underlying construct (measured IQ= true IQ). High validity means that the test measures what it claims to measure. If a person has an IQ 170 but the IQ test assigns that person IQ 160, because this is the highest possible score in that test, then this result is invalid and the test is not a good tool for measuring high IQ.

In practice, the test manual should clarify what is the IQ range, in which the test provides reliable and valid estimates of person's IQ. Furthermore, the authors may provide multiple test versions which differ in difficulty. For instance, Raven's Progressive Matrices come in three test versions: test for normal adults, test for children and mentally impaired individuals, and a test for people with above-average IQ. As you can imagine, the last-mentioned test features higher number of tasks than the normal version.