Using a single pebble, rock or piece of crockery over the drain holes provides for maximum root growth due to a reduced water table level. Even if the same proportions of ingredients as originally used for the root ball comprise the new soil, a soil difference would exist. Organic matter in the original root ball would have leached out. Some compaction also would have occurred in the original root ball. Watering alone causes compaction of soil.
Some gardeners feel the difference in soil type is of little consequence.
This difference, however, can mean success or failure with a plant. If the soil in the original root ball is finer or more compact than the fill soil, water will move into the root ball from the fill soil, often causing a waterlogged condition resulting din root death. If the soil used as a fill is more compact than the root ball, water will move from the root ball into the fill soil. This may result in a dry root ball, again causing the death of the houseplant. The problem is solved by breaking the root ball up prior to potting.