yum upgrade v anaconda upgrade differences

[David Timms]

On the yum list a question was asked which intrigued me:
Why can anaconda manage to upgrade a system, when yum upgrade can't.

The postulation was that anaconda is cheating (ie running --nodeps 
installs). This would allow it to complete an upgrade where dependencies 
lead to unavailable packages that are not on the dvd, but are in the 
complete Fedora, and or non- fedora repositories, that are not available 
at upgrade time.

Is that why it can work ?
Or what are essential differences between an anaconda upgrade and yum 
upgrade ?

DaveT.