Shot placement!!!
Hit bone, get explosive expansion. Miss bone, there’s not much in a woodchucks neck to get massive expansion. Nick a vertebrae, and he’s very dead, but bullet doesn’t hit something substantial enough to generate expansion.
I got similar performance from a CCI Minimag on the last one I shot. Similar shot placement.
In 1993 I went hunting with my two brothers and nephew in Montana. I had a batch of bad Hornady 117’s with my .257Roberts. 7 shots at mule deer, zero hits. Easy 150-200yd shots. Checked at 250yds, zero hits on target. I’d taken 100rds of 85gr Nosler B.T.’s for a planned P.Dog shoot. Fired 3rds on same target, 1.5” cluster at point of aim. Next day, I dropped a 200+lb muley doe at paced 378yds. Dropped to a spine hit. 3” exit wound. Two days later, shot over 60 p-dogs with remaining 96...
The 55gr BT is wildly popular for shooting deer in W. Ga and E. Ala. They’re a surprisingly tough bullet. (IMO, it’s the heavy solid base).