Because containment within the airspace will vary with the number of degrees the aircraft needs to alter heading**. More degrees to turn then more room needed to complete the turn - the aircraft will be further displaced from its original position.
Compare an aircraft altering HDG by 180 deg to the same aircraft altering HDG by only 10 deg. Which one will be positioned further from the starting point (albeit pointing in a different direction) after completing the turn compared to just prior to starting the turn?
**also aircraft speed & wind effect - both of which are considered in the design of the procedure.