You are assuming two different starting situations. In your repost scenario you already have an auction posted, but in your undercut scenario you don't.
When you make an auction in the new system undercutting for small insignificant amount like 1c doesn't make any difference. Only situations where the undercut makes a difference is where the undercut is deep enough that it makes someone who would normal not buy the item want buy the item, or make other people not want to post their items for that price.