Signatures are stored once in the user table and then when the post is called they are shown as they are at the time of viewing the post. This helps to keep database size as minimal as possible, since if it added the sig to the end of every post as of the time of the post, the database size would get larger faster. Plus, the search tool would be disrupted by the multiple occurrences of each sig. Many keywords would be matched up to people's posts just because of their sig and it would make your searches less efficient.