There are a thousand potential reasons for minor differences, so any hypotheses offered here would be pure speculation. However, it's unlikely that the SONAR version or possible version mismatches of plugins/instruments are to blame.
When investigating such matters, the first thing I do is narrow the suspect list. Export your project as a 32-bit wave file and then import it back into the same project. Use the solo buttons to switch between the in-project mix and the imported file. If differences are heard, use analysis tools such as SPAN to determine what's different (spectrum, levels, average RMS or dynamic range). If no differences are heard, then there is something about the WaveLab environment that's altering the sound or altering the way you hear it.
To eliminate plugin behavior that might change during export, try exporting the wave file with all plugins bypassed. Yeh, it'll sound pretty awful, but it should sound the same after importing back into the project.