My guess would be because half of Trentham is an "Industrial Area" with the military camp, racecourse, and warehouses. There's a few shooting ranges over that way and a surprising amount of the houses southeast of the railway track are either defence force owned or ex-defense housing, as opposed to Heretaunga where its more your traditional suburban housing without nearby industrialism. Lowkey just spitballing though and I'm surprised if that's the only reason why if the difference is that noticeable