The depths, widths, and magnitudes of the 410-km and 660-km seismic discontinuities are largely consistent with an isochemical phase change origin, as is the observation that the topography on these discontinuities is negatively correlated and significantly smaller than predicted for chemical changes. While most thermodynamic studies of the relevant phase changes predict greater topography on the 410 than the 660, recent seismic studies suggest greater topography on the 660. The seismic results are consistent with some recent thermochemical studies which suggest that the Clapeyron slopes of the perovskite-forming reactions exceed in magnitude those of the spinel-forming reactions; however, we have reexamined the relevant Clapeyron slopes in light of other, more recent, experimental studies as well as the requirements of internal thermodynamic consistency. We conclude that the bulk of the evidence indicates a greater Clapeyron slope magnitude for the 410 than for the 660. Thus the recent seismic results are unexpected. One explanation might be that lateral temperature variations near 660 km depth exceed those near 410, consistent with a model of the 660 as a thermal boundary layer. An alternate interpretation, which requires neither a thermal boundary nor metastable olivine, is that the 410 does possess greater topography but is simply less visible seismically than the 660. This latter idea, and recent short-period observations of P’410P’ seismic phases in conjunction with an elevated 660, is consistent with thermodynamic modeling of subduction zones illustrating the extreme broadening of the olivine α → β transition in cold slab interiors and, conversely, its sharpening in regions of high temperature.