Entirely possible and normal, if you have specific needs for multiple JVMs. Due to running legacy software, I have specific versions of 1.5, 1.6, and 1.7 on my development machine, and two versions on one production machine.
What you're probably asking though, is why didn't upgrading one upgrade the others? Well, quite often software installs will include the JRE version they were tested on, and store them in a subfolder of the application directory.
I would recommend not worrying much about outdated JVMs that are in the latter situation. The ones you should be most concerned with are the JVMs accessed by your browser plugins, since they expose you to already-patched and even unknown vulnerabilities (for now, turn off Java in browsers, and maybe forever, heh).
You CAN try upgrading the various JVMs and test whether those applications still work, but there's probably no reason to do so, other than performance (for example 1.7 smokes 1.5 in speed, about 2:1).