The ROS size in storage_containers doesn't match with OS

[wqhhust]

In the data dictionary, it reported 3M is used to store the data. but in file system, the file is only of size of 300K. 10times difference. is that a bug in the dictionary?
select storage_oid,used_bytes from storage_containers where storage_oid=49539595929845205;
storage_oid | used_bytes
-------------------+------------
49539595929845205 | 3028633
(1 row)

$ pwd
/data/205/49539595929845205
$ ls -l
total 344
-rw------- 1 test test 333434 Nov 5 23:55 49539595929845205_0.fdb
-rw------- 1 test test 2296 Nov 5 23:55 49539595929845205_0.pidx