The zero therefore usually coincides with the lower of the two low tides occurring in a day, but whether it's the average case or an extreme case depends on exactly which definition was used. When the water tries to follow the moon, it runs up against a lot of obstacles, including its own inertia, the shape of the coastline, and the resonances that are set up by the continual tidal motion. In some cases the tides are fighting a permanent current, e.g., going up a river, and this slows down the tidal crest. The result is that the tides at any one place at any given time don't have a whole lot to do with the moon any more. That is not necessarily a problem.
Some places really do have four high tides in one day, and other places only have one.
These extra tides can be arbitrarily close together. Official predictions might omit them, but XTide faithfully reports all maxima and minima that it finds. This situation can be corrected by replacing the offending data set with a proper subordinate station definition in offsets.xml. The standard simple answer to this question is that the water on the side of the earth opposite the moon bulges out due to decreased lunar gravity in the same way that the water on the side of the earth nearest the moon bulges out due to increased lunar gravity. This is counter-intuitive in that one might expect all of the water to just rush over to the side where the moon is.