Thank you Davide, Glenn, for looking into it. I'm sorry for the noise. I was wrong to attribute some behavior to fuzzy checking. There was a place in my list checker that was using the string method on an individual answer, and this trickled down to where I was stumped. The string method was of course turning something like 0.3750001 into 0.375, so the individual checker did not see the 0.3750001. At least I understand now the root cause of my issue.
Later today I will keep working on the project and see if I can get my list checker to pass an unparsed component from a student list answer to my individual checker. Either that, or I can now imagine alternatives to what I am trying to do.