Ext4 file systems created on RHEL6.4 and RHEL6.6 have different sizes

Issue

Ext4 file systems created with the same method on RHEL6.4 and RHEL6.6 have different sizes. On RHEL6.4 the number of 1K blocks is 63461 and on RHEL6.6 it's 59365. These file systems are created on top of a 64MB logical volume.

RHEL6.4$ df -k /tmp/test
Filesystem           1K-blocks  Used Available Use% Mounted on
/dev/mapper/vgl_root-test_2
                         63461  5385     54800   9% /tmp/test

RHEL6.6$ df -k /tmp/test
Filesystem           1K-blocks  Used Available Use% Mounted on
/dev/mapper/vgl_root-test_2
                         59365  1289     54800   3% /tmp/test