Rigging anchors for balanced load 

because, as he says, you can massively alter the loads with only a tiny shift in the bisecting angle. this makes it less consistent, and give rise to the sudden risk of spiking up to a sudden huge load on one anchor. the larger angle does render the anchors subject to such sudden fluctuations. he's not saying uneven stresses don't occur in the 30-90 range when the bisecting angle is not central; he's saying to be wary of tighter angles because uneven stresses have bigger consequences with less change.