1. That different mech chassis have "good armor" or "bad armor" levels.
2. That different mech variants have different levels of armor.
The confusion largely comes from the fact that mechs are bought as their "stock" variant, and these stock configurations never have maximum armor. This makes it seem like one mech variant has "more" armor than another. But these pre-built armor levels are not important, only the max armor level is, because the first thing you do when buying a new mech is optimize its armor allocation.
The fact is that all mechs of the same chassis type have the same max armor. In fact all mechs of the same weight have the same max armor. Max Armor is directly correlated to mech weight, with a slightly lowering armor to weight ratio as mechs get heavier.
Consider these charts.


As you can see, the heavier the mech, the more armor it has. It's as simple as that. The Hunchback, Centurion, and Trebuchet are all 50 ton mechs, and therefor all have the exact same Max armor and armor per location allocations.
This chart also shows that while max armor goes up with tonnage, the armor per ton ratio goes down. The only exception to this is the 40 ton Cicada which for some reason has more armor per ton than the 35 ton mechs.
I hope this clears things up for anyone who was confused.