Some space is necessary in the container to accommodate the changes of the content volume. The content volume has not been reduced.
Most of cosmetic products have some space on top of the container in order to accommodate volume changes by increase of temperature due to the season or storage conditions. This space acts as an air cushion to help avoid overflow of the product or breakage of the container. Be assured that the volume contained is the correct amount.
There are some cosmetic products for which the quality cannot be maintained if transparent containers are used.
A transparent container is useful to check the remaining amount, but some products must be kept in an opaque container.
For example, there are some cosmetic products containing a substance in which the quality changes by extended exposure to light. It is difficult for these products to maintain their quality in a transparent container for an extended period.