Itss because you get more often good xp tasks/hard tasks. Out of 100 tasks I had like 10 scorpion, skeleton, rogue, bandit tasks, also did 20lvl dark warriors instead of killing 100s to gain less xp. Every 10 task you get extra 100 slay pts. Every 50 you get 350. When i did my 100th task i got like 625 pts.
10 tasks - 350pts. Its all about luck with tasks. If you keep getting 1lvl tasks you wont much xp but points are the same