Reason for bigger or smaller number of rows than expected with mysqlslap

I did a simple insert operation with mysqlslap as follow,

mysqlslap --create-schema=test_crypt -u root -pmysql --delimiter=";" --query="INSERT INTO reference VALUES ('Jean', 1)" --concurrency=20 --iterations=1000

after this I remarked that the number of rows in the table, which I thought should be equal to the number of iterations times the number of concurrent clients, was either a bit larger or smaller than 20000. Is there a reason for that? Is it linked to mysqlslap or to the INSERT operation itself?

Thank you