It’s because of normal Delay and Padding in the MP3. The way around it in Wavelab is to select the Fraunhofer MP3 encoder in the Render Output Format dialog and set, on the Advanced tab, “Embed Ancillary Data for Time and Delay Compensation”. If you’re making VBR mp3, also select “Add File Length and Playback Position Information to VBR Header”.
Then the MP3 will be exact length and in sync when decoded in Wavelab, and in some (but not all) other programs.
You won’t have the problem if you render to wav or flac, or in my experience, ogg if you really need lossy files.