Real Estate Company Offering low interest rates?

I have a client who is lloking to buy a home. He went to an open house and found a property he liked. The RE agent told him he could get a 30 year fixed rate  at 5.5% because he is putting 20% down and is a first time homebuyer. Current rates are around 6.125% on a thirty year fixed. Can anyone expalin how they can offer that rate through thr rel estate company's "preferred' lender?