Engineers test a real gas for use in a high-pressure storage cylinder. A fixed amount of the gas is maintained at a constant temperature of \(320\ \text{K}\), and no phase change occurs over the pressure range investigated. The compressibility factor, defined as \(Z=\dfrac{PV}{nRT}\), is plotted as the volume of the gas is progressively decreased.

Which statement best explains why \(Z\) first decreases below the ideal-gas value and then increases above the ideal-gas value?