The gutters can be clear of debris but if the down pipe has a plant growing out of it then water is not free to flow away from the house.
When there is a root system as shown in the picture, water will slowly make its way through and away as long as it is a slow steady rain.
But if we have a big downpour, it is more likely to overflow and not make its way down the down pipe.
Sometimes there is no plant to give you an idea that the problem is there over the down pipe.
This gutter, in Kingston, had leafy debris sitting over the down pipe. It matted tightly, forming a top over the gutter which would not allow the water to escape down the down pipe.
The next photo shows another plant and root system that was found in the bend of the down pipe.
It really pays to have your gutters cleaned regularly and to keep the down pipes clear. You will have fewer water issues and you will be looking after the life of the guttering. When these root systems grow and accumulate, water gets trapped and will deteriorate the guttering and down pipes much faster.