Not unusual. There are many reasons that could result in such a situation; and since this is a public forum you do not want to post your personal information. The primary reason is very simply that you likely HAVE NOT completed entering ALL of your information --the guesstimate is only as good as the information you enter accurately and correctly.
The ideal situation is to have EXACTLY the correct amount withheld ($0 owed and $0 refund). Unfortunately that is a very difficult amount to predict, and is somewhat of a moving target due to salary increased/decreases and tax law changes.
