there are some apple vs orange type comparisions being made that might be confusing this discussion.
a stave with a latewood natural back or a properly chased ring puts the most dense and strongest wood where the strain is the highest, and most will agree this is the best way to make a bow. But lets compare boards sawn the two different ways.
With a board, a flat grain surface of latewood may in fact not be as thick in some places as others, and the closely underlying early wood may be asked to handle more strain than it is capable of. An underlying imperfection in the earlywood may also be a hidden point of stress concentration.
With a board of vertical grain, the most highly strained surface consists of latewood from each ring. Even thought the earlywood is also present in alternating bands across the most highly strained surfaces, The integretiy of the wood is being averaged between the dense and less dense, and a flaw in the weaker early wood is less likely to cause a splinter lifting on the back
In effect, (if there is no runout of the vertical grain), it offers a more consistent materiel to work with, and may appear to be stronger than the same piece of wood in a flat grain orientation, unless adequate dense latewood thickness can be maintained on the both the back and the belly. Of course if your limb tapers in thickness, earlywood is exposed in varying degrees.