Why is in several cursor ELAPSED_TIME < CPU_TIME + USER_IO_WAIT_TIME?

Oracle rdbms 19c

Why is in several cursor ELAPSED_TIME < CPU_TIME + USER_IO_WAIT_TIME

select 

sql_id, 

plan_hash_value,

CHILD_NUMBER,

executions,

cpu_time,

USER_IO_WAIT_TIME,

elapsed_time,

(cpu_time + USER_IO_WAIT_TIME) "CPU+IOWAIT",

trunc((cpu_time + USER_IO_WAIT_TIME - elapsed_time)/1000000)  DELTA_SEC

from v$sql

where 

trunc((cpu_time + USER_IO_WAIT_TIME - elapsed_time)/1000000) > 10  

order by 9 desc, sql_id;

SQL_ID    PLAN_HASH_VALUE CHILD_NUMBER  EXECUTIONS   CPU_TIME USER_IO_WAIT_TIME ELAPSED_TIME  CPU+IOWAIT  DELTA_SEC

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