Yes, that would be the reason. When you are single and not being claimed as a dependent, the IRS allows you to deduct a standard deduction of $6,300 and a personal exemption of $4,050 from your income. So, if you made about $12,000 then the first $10,350 of your income would not be taxed at all. When you are claimed as a dependent, your personal exemption goes to the person that is claiming you. So, now you would only get to deduct $6,300 from your $12,000, meaning you are paying tax on almost $6,000 of income now, which is why you are receiving less of your withholding back (because it is being used to pay this tax).
