I hinted about that in an earlier post, but thought about it a lot on my last hike. It's just an inverse fulcrum effect. Think about splitting wood with a wedge. With a 90 degree included angle, half of the force from the sledge goes into splitting the wood, half into driving the wedge. With a 45 degree included angle, 3/4 of the force drives inward and 1/4 goes into the split. This is just the same thing, but with pulling instead of driving.
Please excuse the clumsy wording. Some days my neuro damage shows more than others, and today seems like one of those days.