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 increases/decreases and tax law changes.
Many things can affect your refund.
The only way to determine what is happening is to
compare your 2015 and 2016 returns line by line and see where the differences are.
