Let the shopkeeper purchase the table at price \( p \).
To find the difference in their selling prices relative to the shopkeeper's cost price: \[ \frac{x - y}{p} = \frac{1.56p - 0.56p}{p} = \frac{1.0p}{p} = 1 \]
Conclusion: The difference between the selling prices of Amal and Asim is exactly equal to the shopkeeper's cost price, that is, \( \boxed{p} \).