why reduce task can be scheduled before map tasks are 100% completed?

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Hi, everyone.
In class org.apache.hadoop.mapred.JobInProgress, there is a public method:
scheduleReduces(), it will return true if "finishedMapTasks >=
completedMapsForReduceSlowstart"
and then the scheduler can schedule a new reduce task for a given
taskTracker.

but as I konw, reduce can not be started unitl map is 100% completed. Does
anyone can explain it? thanks a lot.