It doesn't seem to make sense because point nine repeating is just plain not a number. You can't have point nine repeating of anything. You can have .9 of something, or .99, or .9999, but what would .9 repeating, as a number, even mean? That's why math doesn't seem to work on it right.

It wasn't exactly as though they were terribly consistent about this- one episode Picard is saying they've eliminated the need for possessions, another episode his family owns a vineyard.