That's how soft limits work.  The implementation is not bad at all.  Please see Disk quota - Wikipedia, the free encyclopedia  The design is such that it allows a user to have a short burst over their quota.  The definition of "short" is up to you. Advisory takes no action other than notification.
I moved a new user and his data onto the isilon system and then created a User quota and the 1-up directory level.  His data was already over the quota limit upon creation.  He has deleted a bunch of data and  "isi quota list shows" he is under his quota, but he still cannot create anything new.  I have su'd to him and get the same response.  I can't find anything to help me...
Quota exceeded for user upon creation