The rules states that, if you have to put a walker mini on the board and there isn't left on the reserve, you have to resolve group tension.
The KS version comes with more walker minis than the retail... So (read title).
Each scenario has a set "Walker reserve" that is created during setup. There are rules that let players use walkers from expansions to modify this reserve to alter the difficulty of scenarios. Additionally, there are scenarios that do not cap the Walker reserve and group tension is instead resolved by the threat track getting out of control.
The additional walkers are taken into account when designing scenarios