This question has been asked many times. If you do a search on this site for "lun space" you'll probably get a bunch of related hits.
The host operating system creates its own file system on the lun and keeps its own "bookkeeping" of space, different from the bookkeeping that the storage system (WAFL) keeps. When you start out with a new lun, both will, of course, show 0% utilization, but over time the numbers will diverge and lun space utilization as seen by the storage system will keep going up, usually to 100% or close. This is normal. If you are also the systems admin, you would be concerned with what the Operating System tells you. As a storage admin, you would be concerned in ensuring that your volumes and aggregates do not run out of space as that could cause your lun(s) to go offline. If you are really adamant that your storage sees approximately the same amount of usage that your host sees, the snapdrive product can do "space reclamation" on the luns, and I believe the powershell toolkit also has a function to do this.
In our company, we don't even look at the lun space utilization on the storage side. Most of our volumes which contain luns but aren't snapshotted are usually at close to 100% utilization (we size the volumes just ever so slightly larger than the lun). The volumes that are snapshotted are a lot more complicated, but that's another topic. Hope that helps.
