Do you think i have grounds to appeal grading on this essay?

I have wrote on essay on a topic and the grade is significantly lower then my other grades and has affected my grade classification for this year. The reason for the low mark according to the moderator is i answered the question over a wider historic period then wanted. However the question never explicitly stated the period which in this case was the 10th century. The module however did mention the 10th century. So to a certain extent i respect that they wanted it to cover that period and it was silly oversight by me, but is it really fair to be marked down when the question itself did not state the period wanted? In addition majority of other essay questions make reference to the specific period in the question...