I knew that time(1) reports the time taken to run something in User Space, Kernel Space. In addition, it also shows the wall clock time. The latter is approximately the sum of the previous two.
When I look at the Figure 15.29 on Advanced Programming in the UNIX Environment. It compares the time taken for several ways of utilizing shared resource. I noticed the Clock is not the sum of the User and System. 
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It would be reasonable if the Clock is slightly larger than the sum of User and System, because the time of context switching is not measured (maybe?), but the second row states that Clock time is far less than the sum of the two.
Either the author is not using time(1) for the benchmark, or I misunderstood the concept.
Can someone explain?