I've lapped it and honed razors on it several times. Based on the amount of use I would have expected only a small bit hollowing in the middle.

Oddly it has shown this bulge pattern before when being lapped. I assumed (as you suggest) that it had something to do with wetting and drying. In the future I may try lapping it dry instead of under running water. (I spritz it when honing rather than soaking it.)

This Naniwa 10K Superstone is 3/4 inches thick rather than 3/8 like those at SRD, so that may accentuate any such tendencies.