The top and bottom of the mattress maintain the same surface as the main part of the mattress to provide comfort for your child. Typically your child will stand at the sides of the cot, not the top and bottom. Also, your child may roll from side to side during the night. This means it is important for the mattress to have reinforced sides. Sometimes your child will end up sleeping with their head near the end of the cot, meaning this needs to be a comfortable surface.