Perhaps the preveous owners releveled the yard when the pool was removed. OUt here people have made a business out of grinding up wood, letting it sit untill it turns black mixing it with a fine sand and sometimes just river silt and then sell it as top soil. A year latter all you have is sand, if they used something like that it may never firm up. Pulling plugs of soil from different areas of the yard might give you a better understanding of what you have. You can make a sample out of a peice of steel pipe you hammer into the ground and then use a slitely smaller wood doewel to push the soil out of it.