@BatDan:
Well, when a series starts out the first eight chapters are unranked. Once it hits Chapter 9, it starts counting the results from eight weeks ago. Hence, Chapter 9 is when a series starts getting ranked.
I'm pretty sure those first eight chapters are just randomly placed, unless it's the first chapter then it's on the cover.
I'm sure Aohige can explain it better.
By the way OmegaSplash, put Assassination Classroom on WMR. It deserves it.
The first 8 weeks aren't completely random, it generally tends to more or less display the overall average polls for the weeks it has been serialized.
That's why a lot of the ones that goes straight to the bottom at 9th week usually slope towards the bottom gradually during the eight weeks (the first chapter usually gets ok polls, and as the following weeks are lower and lower, the average slope downward).
The significance of the first eight weeks though, is that it's "protected" from an immediate cancellation.
It's like the MMO newbie protection period.
They aren't completely accurate, and editorial will control what they want to feature, like when you saw Date-senpai spiked up, it was probably because editorial thought it has potential and gave it a chance to be visible.
Toriko is speculated to have had a lot of help from editorial team seeing the potential and pushing it higher than the rankings in the early days. They are professional editors, they're fairly capable of spotting something that they see potential for franchise popularity.