Quote Originally Posted by Bramborough View Post
The difference is that a land siege cannot go on indefinitely. First, a besieging army takes higher attrition losses than a blockading navy. Second, the city will eventually surrender if there's no sally-out or assault. Third, the AI appears to be much more likely to assault (as attacker) or sally-out (as defender) for a land siege than to execute these actions for a naval blockade. So...the situation resolves itself within a few turns.

With the naval blockade, there is no surrender turn-limit, the AI appears content to just sit out the blockade as either attacker or defender, and the low attrition rate allows them to do so for a very long time.
A port settlement that is under siege from a land army but not blockaded wont suffer any attrition and as far as I know, it will be able to last forever as well or until someone blockades the port.