This is pretty meaningless as a blanket statement, IMO.While there are exceptions to every rule, it is much more common in the metal scene to attempt to play every part and every song as consistently and exactly "correct" as possible, every time. The impact of the instruments locking together precisely in the manner in which the parts were written is generally considered more important than any concept of freshness or spontaneity. A player who tried to "play every tune a bit differently every time" would probably be fired from most metal bands pretty quickly.
Like any other genre, great metal is all about dynamics. "All instruments locked in exactly" is just one tool in the toolbox, and can often be deployed most effectively when juxtaposed by sections that breathe a little more easily.