cpu_cost oracle document 12.2 different between PLAN_TABLE and DBA_HIST_SQL_PLAN

In oracle document, there is different description on cpu_cost column on those 2 views.

Is DBA_HIST_SQL_PLAN table description more precise about cpu cycles? '
In DBA_HIST_SQL_PLAN, the cpu cost are there for each operation, so how can we calculate cpu cycle for the query? is it sum up all those cpu_cost's operation or just take from the top only?

thanks

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