I noticed on my Pokémon Platinum cartridge that I have far fewer Water Stones in my inventory than any other type of evolution stone. I have around 30 Fire Stones and 20 Thunderstones, but only 2 Water Stones.
Now, I'm a fan of Vaporeon, but I don't remember going on a Water Stone spree. Is there any reason that Water Stones would be less common in the Underground? Does it vary by game or player ID? Does it vary by area? Do some games get no stones of a certain type, the same way some Pokémon are version-exclusive to encourage trading?