Is there air in the volume of crystallized sodium hydroxide? I have a container of crystallized NaOH (98%) which has a volume of 500ml. Now, as per wikipedia, Sodium Hydroxide's density is 2.13g/cm3. This should give the weight of a 500ml container of NaOH at 1065g.

However, when measured, the crystalls weighed in at around 560g (minus the weight of the container). Is it because the NaOH is not in liquid or a homogenous crystal? Is the rest of the volume around the crystals air?
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.