There are several examples where multiple liberties cannot be safely reduced by one of the players. Some of these are one-sided dame, while others are such that when the first player plays, the second player must make a defensive move within the seki to restore safety.
What sort of configuration is assumed when saying: "(Note for example that a simple no-eye seki may have more than two shared liberties to start out with, but either player may reduce the count to 2 without danger, so I count this as a "score" of 2 regardless)"