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Powder bed depth
The method of pouring and brewing in batches includes pouring water through a coffee bed. Like other variables, the important thing about bed depth is consistency.
If the coffee grounds are not evenly piled or soaked evenly, the water will form a channel in the coffee grounds, and more coffee will be extracted near the channel than in other places. Water always takes the route with the least resistance, so we tamp espresso for a similar reason. If we don't turn the coffee into an espresso, the water will flow directly to the place where there is more air.
Pouring water too quickly or irregularly will shift the coffee grounds, making them "high and dry". This is when they sit dry or not completely soaked in the filter above the level of water. This usually happens when the grinding is fine, or when you pour directly into the middle of the bed.
Leaving some residue in the brewing process means that not all coffee beans can be extracted, and your method is inconsistent. To avoid this, start with a uniform foundation and consider using pulse filling or stirring to ensure that all foundations are evenly saturated.
Notice how deep the coffee bed is. The shallow layer may allow the water to pass through quickly, preventing a large amount of extraction. Too deep a bed may expose water to coffee for too long and may lead to overextraction.
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