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).
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).