I think that the "bottom line" here lies not so much in nit-picking over 2 knots difference between the correct answer and the "nearest correct answer". 2 knots variation is definately within the "green band" for practical day to day operations.
In sitting for exams, the candidate will typically do his / her working to the highest degree of precision possible, and considerable stress can be caused in an exam situation where the correct answer fits none of the answer choices available.
BTW, drift WILL complicate the issue!
Regards,
Old Smokey