The problem with looking at rates on-line is there is always the fine print. Most rate quotes assume the mortgage is a purchase of single family home, 20% down payment and credit scores of 740 or better.
If your scenario falls outside of their assumptions there may be rate add-on’s that are not included. For example there are tiered rate adjustments depending on credit score and equity in the property, property type (2 family, condo, coop etc.) and occupancy (primary, second home and investment property)and transaction (purchase, rate and term refi or cash-out refi)
The rates published by Freddie Mac come from a survey of mortgage professionals across the nation. It isn’t a great tool to see what rates are available to you but, more of a tool to see where rates are trending. The only way to get an accurate rate quote is to speak to a few lenders that will quote a rate on your actual scenario.
As for your individual scenario, you are in the best possible rate situation as you wouldn’t have any rate adjustments. On a 15 yr mortgage you should be able to get 3.375% without paying points.
I hope that helps,