Well I havent been married that long, but I would think that by age 40 both have probably had all if not most of their major arguments so most common ground has been found between varying matters. Hence, when a situation arises, most likely its been something they've already dealt with so they know just how to meet in the middle and or work out those differences.
Not only that, but the couple probably doesnt even have the energy for those big blow outs anymore which is defintatly a good thing!
Sure couples will still bicker to the day they die, but I think it gets more about smaller insufficient things. Sometimes couples even get bored and or so comfortable that nagging and arguing is a natural way of communicating. Anyway, try not to fall onto that path, sounds like your doing fine and all is normal. Sooner or later, I think you'll both calm down more. For now practice trying not to argue period. Instead have more mature adult-like conversations when a matter arises. As a married couple, you wont agree on everything, but the beauty of it is finding the shade of grey.