It is known that under some circumstances you do not automatically receive the mega stone for your second starter Pokemon (Charmander, Squirtle, Bulbasaur) when you obtain the pokemon from Professor Sycamore. Instead you have to purchase the Mega Stone from the Mega Stone Salesman.
What are the circumstances that trigger this situation?
Best Answer
I'm fairly certain you'll always get the mega stone for the gen1 starter you picked from Sycamore (with charizadite having version exclusive X and Y variants). But the stones for the two gen1 starters you didn't pick are available from the guy in the Stone Emporium.
The phrasing on the website you linked;
May have given you the impression that it's possible to not get the stone for the starter you picked, but it is meant to indicate that the other two stones for the gen1 starter mega evolutions are available elsewhere. Also note that the price of the mega stones in the Stone Emporium goes down the more 'stylish' you are after you've beaten the elite 4 (down to a minimum of 10,000).