Fw,

Without seeing them close up it's hard to tell. I agree entirely with earlier answers and the relationship with weathering. I suggest the same is true here. The two stones must be of different rock or, more likely, the same rock but a softer/harder piece of that strata. It is not uncommon to have many hardnesses of the same geology, from the same locality, depending on the type, formation and quite a few other factors.

What is the main geology in the area?