According to Anders Swahn:

"Think of a river that enters the ocean at high speed. At the entrance to the sea it is likely that you would find tide in the river but never have slack as the river is too fast to run backwards. So you have high tide and low tide and no slack. Now if the river ran a bit slower the current would start to slow down as the tide came in from the ocean and as the tide continued to rise at the entrance it would slow down more and more. Eventually the current would stop (slack) but the tide would continue to rise from the ocean. After a while the river would run backward (upstream). Well here is an example where the slack and high tide does not coincide. Hence I propose that the reason we don't have high/low tide concurrent with slack is that we have an overlaid outflow from the bay due to the rivers and tributaries that feed the bay."

According to Chapman's:

"A tidal current is said to flood when it flows in from the sea and results in higher tidal stages. Conversely, a tidal current ebbs when the flow is seaward and the water levels fall.
As these currents reverse, there are brief periods of no discernible flow, called slack, or slack water. The time of occurrence of slack is not the same as the time of stand, when the vertical rise or fall of tide has stopped. Tidal currents do not automatically slack and reverse direction when tide levels stand at high or low water.
High water at a given point simply means that the level there will not get any higher. Further up the bay or river, the tide will not have reached its maximum height and water must therefore continue to flow in so that it can continue to rise. The current can still be flooding after stand has been passed at our given point and the level has started to fall."