Another bug in the Brazilian UP. (If I understand the code correctly)
In CvCity.cpp
IIRC, the code makes the game thinks Brazil has more oil (because of the sugar), and calculates the yield bonus with the increased amount of oil. However, this increased number is also be used while calculating the yield bonus for every other corp that has oil as one of it's required resources. So the sugar bonus also applies to the Computer corp as well.
EDIT: I see this is only about the commerce. It's likely the same bug applies when calculating the yield-production. Can't find where that is calculated.