Again: if instead of another offer, over that weekend a meteor had landed on the property and left nothing but a crater, would *you* think that you were still obligated to the terms of the unsigned contract? I'm pretty sure that if the seller tried to maintain the offer as binding at that point, you would complain that nothing was signed.
Again: "Highest and Best offer" contains "and Best". A seller might think a particular offer is better for any number of reasons. Some of those reasons might appear objective to a potential buyer.