Use of lower instead of upper case in q7

7.  "Comment va ta jambe ? - ________." How's your leg? - Better and better.My answer “De mieux en mieux” was marked as almost right, the preferred answer being “de mieux en mieux”ie I used a capital letter. Small point, but I need to understand why. To me, it does not make sense to commence with lower case, as the missing words are clearly in response to the question asked and are a separate (albeit incomplete) sentence. Is this just another (annoying, irrational) “French” thing that one must accept as being different from English?  Please can someone explain. Thanks