A negative variance indicates that less of an ingredient was used. Something should have been used, but was not. In this instance, some one is either not following recipes, or using the incorrect portions.
A positive variance means that someone used more than what they should have. If you have a positive variance, look for adherence to recipes, portion controls, spoilage, or lack of rotation. You should also check that the cooks are not using one product in place of another product. This is another variance that needs investigation.
Remember that variances are differences from Ideal, and the more that you can find and rectify, the more money is in your pocket.