@NurarihyonNurarihyon We don't know how the gate detection system works. There might be a threshold. Bear in mind that as has been implied in previous gates, the nature of the gates are changing, escalating. It was previously unheard of for gates to upgrade in nature, nor the increase of frequency of them opening. Plus, they're no longer the big ones that open slowly. These might be factors why the Hunter Association can't keep up.
EDIT: Before I get chewed up, yes I've read the novel, but what I wrote up there is based on everything implied already quite clearly, with guesses that a person who haven't read the novel can reasonably make where appropriate.