Once I started my Waterlust build several years ago I had a similar question after studying the drawings vs instructions in the copious build manual.

What I got back in reply to the query I put to the designer was that the drawings were correct and that the copy didn't fully reflect that; the drawings supplied were from the last design iteration prior to kit production. He went on to add that there are 'in-between' profiles a user can choose depending on their intended use and personal sailing skills.

Tapers reduce weight aloft while at the same time reducing strength somewhat so there's no hard, fast rule as to what's really 'correct' for every builder. Too, a tapered spar will be more flexible than one left squared up so there's an aspect affecting sail shape in how far a builder wants to take tapering of spars.

Ultimately it's left up to each builder to choose their preference for taper.

   Thx. I guess there's no real wrong way to shape the spars, unless they break under pressure. It feels like taking more material out of a 3rd edge is a bit much though. 
happy sailing. 

"... taking more material out of a 3rd edge is a bit much...."

Can be, yeah if the spar breaks at an inopportune moment when you're out & about with it.

Tapering the width pays in flexibility at the aft end of spars where the forces can be less than forward; side to side bears on stiffness that's worth something where a sheet's attached. Farther back the need for stiffness lessens... up to a point.

Stiffness up and down matters maybe more owing to the lift that has to be dealt with when a sail's drawing well and that pull is what's moving the boat forward. Again it's important forward of where the sheet attaches.

All these combined forces are in play as the area of the sail increases forward of its sheet attachment point and that the mast load's grows the closer the forward area of its sail is to it. Between mast and sheet the sail's force is what moves you forward; tapering done too far forward may affect how much force can be carried into the mast.

Tapering is a tool that helps control both the forces being applied to the hull underneath those spars at the same time as the shape of the sail attached is being tuned by the crew to achieve an optimum shape for conditions.