Mortgage Rates: Purchase vs. Refi

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please take a look at this graph. It shows a 20 year fixed rate for purchase and for refinance. in the past 7 months the refinance rate is slightly higher than the purchase rate (the previous 5 month the rates were almost identical)

What is the reason for different rates if you buy a house or if you refinance?

http://www.bankrate.com/funnel/graph/Default.aspx?cat=2&ids=293,31&state=zz&d=365&t=MSLine&eco=-1