Sometimes fractional scores for questions result in a question or total score with six decimal places. I can't reliably trigger this, but have reproduced it on all three builds.
Example on http://aam-dev.stanford.edu:16080:
Site PHYS 100
Assessment "4.18 regression test"
If you view the total scores and the question scores (particularly for q3, the matching question), you'll see the six decimal places.
This appeared spontaneously once, and I produced the others by modifying the question scores for individual submissions.
Nothing helpful looking in the logs.