The issue is moisture. The less moisture that a cutting/plant loses, the less it has to take in to make up for the loss. The more leaves you have, the more surface area to lose water from, the greater the loss of water. In situations where the leaves outpace the growth of leaves early on, intake may not be able to keep up with outgo.
By increasing the humidity around the plant, you decrease the moisture loss from the plant, reducing the need to intake additional moisture. By covering/containing the plant in a soda bottle or other enclosure you allow the plant to shed moisture into a small, closed system. Eventually the system reaches an equilibrium point where the moisture in the container is sufficient to preclude further evaporation from the leaves.
We don't necessarily need to add moisture, or know what the percentage is. The system will reach a natural balance. Taking the cover off, or not using one, makes the "system" bigger, requiring greater amounts of moisture to reach equilibrium (due to larger volume of air).
If you sweat/perspire/glow in a humid environment. such as the South, you do not dry off very quickly (if at all) because the air is already saturated with moisture. Conversely, in Arizona where the humidity might be 2-3-4%, you will dry off very rapidly. In Arizona, you will need to drink a lot more water because it will evaporate right out of you, even if you don't sweat.
The plant behaves the same way that you do
Eventually the plant grows/matures to a point where it can take in sufficient moisture, with a better developed root system, and the issue is minimized. Compare yo0ur own baby, needing feeding every few hours, and not being able to do much of anything for itself. Eventually the baby grows into an adult and eats 3 meals a day, and can go for several days without any food, if necessary.