The images are just snapshots of the current system at that time, while apt-get update/dist-upgrade will make sure the latest available software is installed.
The images are supposed to be for your initial install, then use apt-get upgrade to keep up to date.
The only way these will differ is when the Foundation decide to make changes to the list of software that is installed, or change the configuration for some reason. Even so, I'd stick with apt-get for updates, otherwise you are gong to have to start from scratch each time. One of my Raspis was set up using one of the first Raspbian images and has the latest versions of everything it needs on it.