Most every house has this some worse than others. What is happening is what I call the sponge affect. During the summer or dry months, the soil becomes dry and condenses because the lack of the molecules H2O. So, the foundation drops a bit. Now the rainy season comes and the dirt fills up with moisture and the soil expands, so does the foundation. So the cracks close up.
Its a game to play, but you will not fix it for good unless you apply foundation jacks to make this all stable.
What I do is to fill in the cracks with expandable white paintable caulk in the summers when the cracks are open. then when they close up the expansion of the caulk will also close up.
You can minimize the cracks by watering your foundation during the dry months to keep the moisture content even all year. This will help drastically.
In Texas, every home owner has his home outlined with black soaker hoses 16 inches away from the slab foundation and during our summer droughts we water the foundation as needed and all sides evenly. Its a balancing act, BUT it works.