They all do it! Every brand, every model, every hide. The leather has some elasticity to it (stretches, just like skin) and you're seeing the result of that. Thick leathers are a little less prone to the pocketing but not my much. They smooth out easily with just a rub of the hands. If it gets severe over time you can put a hair dryer on the area and that will tighten it up, or remove the internal cushion and have it 'wrapped' so that it fits the leather casing much tighter, and have some crown added to the top of the cushion.
If you get a chance, post a photo of the 'pocketing' on your chair seat before you smooth it out and let me take a look to see if it might appear excessive. If it is, B-Y will wrap your cushion for you at no charge (as described above).
Duane Collie
Straight answers from thirty-six years in the business.
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