My logical volume reports a size of X while 'df' reports a smaller size. Why?

Issue

• Following a logical volume extend using lvresize or lvextend my filesystem size does not match the newly resized logical volume.

 # lvscan -a
  ACTIVE            '/dev/vg_example/lv_root' [6.00 GiB] inherit <------\
  ACTIVE            '/dev/vg_example/lv_swap' [1.00 GiB] inherit        |
                                                                        |
 # df -h                                                                | Different sizes
  Filesystem            Size  Used Avail Use% Mounted on                |
  /dev/mapper/vg_example-lv_root                                        |
                        3.6G  3.5G     0 100% /        <----------------/                 
  tmpfs                 7.8G  100K  7.8G   1% /dev/shm
  /dev/vda1             485M   54M  406M  12% /boot