First, it matters how much torque you produce.

Let’s say the 90’s are stiffer than the 70’s, if you aren’t strong enough to flex the 70’s, then having a stiffer sole won’t matter.

Also, the difference could be in weight savings, not stiffness. And the 90’s being so light that they are less stiff. Like the difference between cromoly and carbon saddle rails or handle bars.