[Freak_NL]

You misinterpreted the wiki page. That tag (like several others) explicitly supports user-defined values. This is useful, because you cannot define a complete set of values for something as open-ended as a way's surface. Innovations happen, and sometimes odd things are used to pave a way for a variety of reasons (art, tourist appeal, experiments, etc.)

So that table there lists all common values covering 99% of the use cases, and finally links to TagInfo for all values in use. That 'all commonly used values' bit is slightly misleading, because TagInfo lists all uncommon values, but it true in the sense that any common value missing from that table will be listed in TagInfo (being derived from the actual database).