There could be many reasons but when the IRS issues a refund smaller than what yiu reported on yiur tax return they always follow up with a letter giving the reasons 2-3 weeks after the refund was issued. If the letter did not get to you then you should call the IRS to get the reason. (note that state refunds are issued by the state, not the IRS)
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**