I agree with Jimmy, eKretz and Stefan - some of these stones do exhibit expansion/contraction.
My shapton glass stones did it. No point in flattening them after a honing session as next morning they would be out of true again. Oddly, not in the same places though, i.e.: where one was high in the middle one day, it would be high on the sides another day. Sometimes there was a series of low and high patches.
My superstones all had this differential movement too. Easier to lap it out on these as the stones are 'soft' in comparison to the glass stones.
My Shapton Pro is always low in the middle - but at least it is consistent. Part of it could be hone wear, but when I lap it and hone a flat thin tool blade in just the middle of the hone - say an inch wide or less, the next day when I lap it all the middle is dished again.
My Choseras do not do this - they stay flat for quite some time and only honing on them dishes them, but the 10k is prone to crazing all over the surface - replaced once, the replacement does the same thing.
My Naniwa 8k Snow White - a very hard stone, somewhat like the SG20k - also exhibits some differential movement which is a chore to lap out because of its hardness.
My SG20k is much like the snow white - differential expansion/contraction, but is harder to lap by a fraction.
The room I use is more or less of a constant heat and humidity, so I guess its a case of YMMV.
Regards,
Neil