I feel like I was shafted by my lecturer

So my first assignment for this academic year was to write an essay and the maximum mark was 40. The assignment brief said "Write minimum 500 words under the following 3 headings: What have I done in the past? Where am I today? What are my plans for the future?"

Now I assumed that they meant I should write 500 words under each heading, which I did and wrote just over 1500 words. I get my essay mark back today and my mark was capped at 25 (which normally only happens if you turn it in late) with many marks taken off because I wrote way over the 500 word limit. I must just be an idiot but I feel like the English in the task alluded more to me writing 500 words under each heading rather than 500 in total, what do you guys think, do you agree and should I complain or am I stupid?