Yeast strains do help certain flavors stand out, as you noticed in the the first W-34/70 batch and the first WY2206. Not to say that the manufacturer of your chocolate malts or degrees of roasting may have changed (i.e. used Thomas Faucett Pale Chocolate in batch 1 and Thomas Faucett Dark Chocolate or Briess Chocolate in batch 3).
As for the second batch that has 1.5% Chocolate and 1.5% Carafa II, I bet it was the Carafa II that made it not seem as roasty since the Carafas are kilned to not give off roasty, astringent, or chocolate flavors.
Hope that helps.
Bottled: Oatmeal Porter, Double Chocolate Chipotle Porter
Kegged: IPA, Red Rye