La question avait été posée sur le site officiel de wotc.
November 7, 2007
Q: Why do some creatures like Mistform Ultimus or Chub Toad not get their unique abilities errataed when they become keyworded abilities in later sets?
–Kaj, Sacramento, CA, USA
A: From Del Laugel, Magic senior editor:
Keyword abilities come in two varieties:
* Evergreen keywords like flying, first strike, lifelink, and reach can appear in any set. Every Magic player should learn what these keywords do. If you're teaching someone how to play, be sure to give him or her a copy of the Keyword Cheat Sheet, available on the Magic Rules Page. (I'm going to start taking copies to the Employee Prereleases here at Wizards.)
* Block-specific keywords like buyback, storm, dredge, and evoke only appear in certain sets or blocks. R&D will occasionally bring back popular or versatile keyword abilities. For example, both Urza Block and Onslaught Block had cards with cycling. In general, though, you won't see a singleton card with a keyword from another block. That's why Mistform Ultimus doesn't have changeling, Chub Toad doesn't have bushido, and Tawnos's Coffin doesn't phase things out.
The Time Spiral block blurred the distinction between the two kinds of keywords. Almost every block-specific keyword that could return in that block did. If you started playing with Ravnica, then dredge has been in every block you've known! In the same block, five new evergreen keywords were introduced: flash, deathtouch, lifelink, reach, and shroud. And until the Lorwyn release, deathtouch appeared on fewer cards than delve!
As time goes by and more sets are released, the distinction between evergreen and block-specific keywords will reemerge.
En clair, c'est contre la politique de Wizards d'errater une carte unique d'un ancien bloc pour faire apparaitre un mot clé introduit par un bloc ultérieur.