Percent Certainty is also called Pseudo R-squared. This fit statistic is dramatically affected by the number of classes you request. If you request just two classes, the % Certainty will be relatively low. If you request more classes, especially beyond 10 (for argument sake), the fit will be relatively high.
If you haven't already done so, I recommend using HB to assist you in cleaning the data of random responders, speeders, etc. prior to running latent class.