I just went under contract on a house, but the sign in the yard still says "For Sale”. Can the seller accept a better offer if one comes along before closing? Why don’t the signs say "Under Contract" now?

Once you have an executed contract, another offer cannot upset your offer unless your contract has a contingency that specifies this. The most common contingency is that the buyer’s current house has to sell. If another offer comes in that the seller wants to take, then the seller would come to you and ask you to remove your contingency. If you cannot, the seller usually has the right to accept the other offer. If no such contingency is in your contract, your contract is safe from other offers. Many Realtors choose not to change the sign status when their listings go under contract.