I am probably wrong, but it seemed to me that the stone was a bit softer when new. Now that it has worn a bit (just slightly under the 250 mark) it seems to be more stable.
On the other hand....it may have nothing to do at all with stone hardness and everything to do with new user inexperience.
On the other hand....it may have nothing to do at all with stone hardness and everything to do with new user inexperience.