Numbers are abstractions that exist for the benefit of the players, and is not something that is known to the characters in character. A gnome wouldn't know "this poison deals 7d6 Poison damage", he'd know "this poison is a very lethal poison".
Furthermore, if he is even knowledgeable about dwarves, he wouldn't know that "Dwarves have Resistance to Poison damage", he's know "Dwarves have strong constitution and are likely survive lesser poisons".
What do you give somebody who might survive a weak poison? You give them the very lethal poison.
