Why does an OR blade/shaft path need to remain vertical but a kayak or surfski blade can be at an angle? With all things being equal, the pressure on the back of the blade is the same whether vertical or slightly angled.

I have long arms so it's easier for me to keep my arms straighter if I put a slight angle to the blade/shaft, allowing me to use my larger torso/back muscles throughout the stroke. Staying vertical and drawing straight down next to the boat causes more bending of my lower arm thus using the smaller muscles of my arm.

I know what your supposed to do, but haven't found out exactly why there is a difference between the two blade/shaft angles.