That's because the XMI file itself is screwy.
It seems the author edited it on another system (probably Mac or Atari), then just transfered the data without any sync to a PC by setting the recording computer's tempo to 100.01 and then just recording it, no matter what the original tempo was, THEN converted it to XMI. It's the same thing with Wing Commander 1 MIDI files, and Realms of Arkania II's XMIs.
Therefore, I suggest to test your converter, you use XMI files from some other game. Try this file from Ultima 8
--- it has tempo changes within the file and lots of volume slides, mercilessly exposing any error in your tempo handling. If you do it properly though, it's guaranteed to fall into the measure-beat grid.