The default parameters for the base_local_planner allow for a fair amount of freedom when tracking a global path. To try to get the local planner to follow your global plan more closely... I'd take a look at the following parameters:
http://www.ros.org/wiki/base_local_planner#Trajectory_Scoring_Parameters. In particular, you can probably up the path_distance_bias a bit. Hopefully, playing around will get you where you want to be. If not, let us know the issues.
I know some other people have run navigation on the creates, but I don't have much experience on that platform so I'll let them chime in on their experiences.