Hi Drew,
We're happy to take a look at the specific linking you're doing and the gram weights you're coming up with. Sounds like you may be choosing a different (either more or less accurate) nutritional item and/or amount than the one that MC comes up with.
For the same nutritional amount, and the same USDA ingredient, you should get the same value. We work off of the USDA's SR16 database; not sure which one MasterCook is using.
As background, here's more information on why this "training" process is necessary for the nutrition-linking. Since BigOven lets you import 170,000+ recipes with ingredients written in all kinds of different ways, we had to engineer a way for people to be able to handle all the different ways that people write an ingredient to a single nutritional record.
That is, we needed BigOven to be able to pull down any one of the recipes from the archive, and let the user associate, say, "Scallions" and "Green Onions" and "Small green onions", etc. to the right nutritional item. MasterCook doesn't have this kind of web-based integration and sharing, and thus has a bit of data-luxury there at the expense of a major feature (web-based sharing and common ratings/photos, etc.). They can force everyone on the front-end to "normalize" their data entry. With BigOven, it's deliberately more flexible in letting you create shorthand ways of writing ingredients, and then linking them up. BigOven will learn these over your use of the product, however, and it won't ask you repeatedly to match the same ingredient name with the shorthand you (or one of the other 500,000 members) may use.