"clearly not correct for my model"
what makes linear correct and nonlinear incorrect?
have you tested a model with known results? this would help understand better if it is your expectations that are wrong or the software is wrong. right now it is a 50/50 chance until you have something to compare it against. since we don't know anything about your analysis, it isn't possible to make a guess if you're right or if the software is right. but theoretically both should be the same if the loading is exactly the same.
i believe this topic has come up a couple of times and that in the older version of solidworks there was an issue with this. so posting your model might be good to have someone run a newer version.
if you're going to a university/college right now, they should have the latest and greatest version to check against.