oceanography - Why is there a daily inequality for low tides?

The water for high tides needs to come from somewhere; the mean sea level should stay approximately constant, as long as wind is neglected. That way tides are a kind of oscillations.

In the first of the two diagrams a low low tide is followed by a high high tide, and a high low tide is followed by a low high tide. That way the mean sea level, averaged over one low tide - high tide period remains constant:

In the second diagram you need to average over two low tide - high tide periods to get a constant mean:

The first diagram can be explained by the superposition of semi-diurnal tides, while
the second diagram needs an additional diurnal constituent. Depending on the amplitude and phase shift of the diurnal constituent relative to the semi-diurnal constituent we can get different resulting oscillations.

More about tides and their constituents.