It's possible for games to have over 100 achievements, which could lead to it rounding to 0% even when they have an achievement, or to 100% when they still have

It's possible for games to have over 100 achievements, which could lead to it rounding to 0% even when they have an achievement, or to 100% when they still have one missing. People won't like this, so perhaps Steam put in some logic to adjust the numbers, and it is also affecting your case. EDIT: But I would have gone for ceiling(99*achievements/total), which does give 57% in this case.