Why package reinstallation would cause to change md5sum of files which are provided by package?

Issue

• Package reinstallation would cause to change md5sum of files which are provided by package as following:

# md5sum /bin/bash
dea623f25c87d86b8c63af707322fed1  /bin/bash

# yum -y reinstall bash

Installed:
  bash.x86_64 0:3.2-24.el5                

# md5sum /bin/bash
dbb9e394d5e2017fb460c4f7d8618f59  /bin/bash