If you start with the hard problems and then do the easy problems, your score will be less than when you started with the easy problems. So the system is rewarding you to do the safe, boring problems.

Today, this worked out really badly for me because I failed the hard problem, problem D. So I only got points for A and B. So my score was less than some of the participants who only solved problem B. http://mirror.codeforces.com/contest/672/standings/page/124

If I solved D, I would have got less than 2000 points. That's less than the participants who solved A, B quickly and then took the rest of the 2 hours to solve C. D is a much harder problem than C. http://mirror.codeforces.com/contest/672/standings/page/20

And why did I fail D ? I used cin instead of scanf() and got TLE !