Issues with alter table InnoDB

[ayran abcdef]

System-specs:
-- MySQL Version 9.10 Distrib 5.5.60
-- CentOS Linux release 7.5.1804 (Core)
-- innodb_file_per_table

Problem:
table status of some innodb tables shows "data_free=4194304".

optimize table [tablename] nor alter table table_name engine='InnoDB' changes data_free to 0 either.

My global variables are set as below in my.cnf file:
innodb_file_per_table=1;
innodb_file_format=Barracuda
innodb_file_format_max=Barracuda

Any clarification would be much appreciated.