Below is the climb performance from the 1978 Cessna 152 POH, which I've plotted as climb rate versus density altitude. It shows that higher pressure altitude gives decreased performance even when the density altitude is approximately the same. It also gives a different climb speed for each pressure altitude, so it isn't exactly comparing apples to apples. But it identifies this as the best climb speed so it still claims that performance is degraded with higher pressure altitude independently of density altitude.
Is this trend accurate? If so, how and why does pressure altitude affect climb performance for a fixed density altitude?