The differences are mainly that with ergonomic chairs, the seat height, armrest height, lumbar support and recline tension can be adjusted to maintain the natural lumbar curve of the spine and adjust the user's position within the chair appropriately. Most office chairs do not have enough adjustability to provide appropriate support for different users.
Adjustable chairs allow for different body types and positions, and if adjusted properly may reduce back, shoulder and neck discomfort associated with permanent use of a chair.