Why does 'e2fsck -n' error out while being executed on a mounted filesystem?

Issue

• Why does 'e2fsck -n' show wrong inode/block counts while being executed on a mounted filesystem?

fsck from util-linux-ng 2.17.2
e2fsck 1.41.12 (17-May-2010)
Warning!  /dev/sda1 is mounted.
Warning: skipping journal recovery because doing a read-only filesystem check.
/dev/sda1 contains a file system with errors, check forced.
Pass 1: Checking inodes, blocks, and sizes
Pass 2: Checking directory structure
Pass 3: Checking directory connectivity
Pass 4: Checking reference counts
Pass 5: Checking group summary information
Free blocks count wrong (465259, counted=451937).
Fix? no

Free inodes count wrong (127978, counted=127977).
Fix? no


/dev/sda1: ********** WARNING: Filesystem still has errors **********

/dev/sda1: 38/128016 files (2.6% non-contiguous), 46741/512000 blocks