Is there air in the volume of crystallized sodium hydroxide?
Imagine a container with sand; it may seem "full", but surely you may pour in some water, and it will fit in the cavities. Same thing here, except in case of $\ce{NaOH}$ I'd rather not check it directly with water, for the response may be quite intense.

$\ce{NaOH}$ commonly comes in granules; if you'd have one big solid brick inside your container, that would be another story.