Besides, whether you're able to lock in 5.0% or 5.18% . . . both rates are considerably lower than 6%.
I don't have an exact answer for you but rates have been around 5%. One way that the bank may be lowering the rate is by charging points to buy down the rate. A point is 1% of your loan amount. By paying extra points now you can lower your rate to even less than 5%. I had a client purchase a home last month with a 4.675% rate. Ask your broker why the rate is 5% but it could mean that you paying points.