Are the Ubuntu ISO images updated From release .ubuntu.com

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http://askubuntu.com – Just got idea from this(may not be related though) question however. Are the ISO images from the official site updated with updates in Core Ubuntu system , like Kernel Updates , desktop Environment Updates(unity), i mean Updates of BASE system including X-org, Office suite, Package Manager , Update manager or Gnome Base Modules, those released in update Branches like precise-Updates branch. The reason i am asking this is , if i download the ISO image of Ubuntu 12.04 Say after two or three months from release , i have to do an update of approximately 200~300 MB's size. So why are these ISO (HowTos)