Even after a tree is felled, sawn, and dried, wood remains a hygroscopic material — it absorbs and releases moisture from the air around it. This means it will gently expand in humid conditions and contract in dry ones. This is not a defect. It is wood being true to its nature.
Understanding this helps you choose the right placement, care, and finish — and helps you appreciate why small seasonal changes in appearance are a sign of real, honest material, not poor quality.
Every feature you see in natural wood — the swirling grain, the dark knots, the occasional crack — was formed by the tree's own biology, responding to its environment over decades. You are looking at compressed time.