Say that the assignment that you are grading is ambiguous, so that you are not sure what the students meant. One interpretation leads to a correct solution, the other one, to an incorrect solution. What to do then?
The Murphy's law approach: if something can go wrong, then it will be wrong. The student kept the writing ambiguous because he or she was not sure which specific interpretation was correct. Then, it should be counted as wrong.
But doesn't that contradict the mustard seed approach? Which approach will be taken then depends on ... on the grader's philosophy? on the grader's mood? on what he or she had for lunch? on some random coin flip, but decided consistently across all graders?