Comparing isotropic solids and fluids at either imposed volume or pressure, we investigate various correlations of the instantaneous pressure and its ideal and excess contributions. Focusing on the compression modulus K, it is emphasized that the stress fluctuation representation of the elastic moduli may be obtained directly (without a microscopic displacement field) by comparing the stress fluctuations in conjugated ensembles. This is made manifest by computing the Rowlinson stress fluctuation expression K-row of the compression modulus for NPT-ensembles. It is shown theoretically and numerically that K-row vertical bar P = P-id(2 - P-id/K) with P-id being the ideal pressure contribution.