Rather inconsistency than a bug. Electrolyzing Cassiterite consumes only 1 dust while output is 1 Tin and 2 Oxygen. Same also goes for Garnierite, Uraninite, Yellorite and Chromium Dioxide (same issue with its Roaster recipe).
All the Ore Materials that are simply “Material + Oxygen” count as a 1:1 Ratio, that’s why. See it as a Balance thing to make the most direct Ores more profitable.
And are you sure it does not consume Carbon? Also have you checked NEI (hover with mouse over carbon) to see if the carbon is even supposed to be consumed?