It's actually due to the cell count.
Actually, it's due to how dry yeast is propagated. It takes approximately 3.8 trillion yeast cells to reach saturation (a.k.a. maximum cell density) in a 19L (5-gallon) batch; therefore, the difference between 100, 200, or even 400 billion cells is in the noise. What matters is yeast health, and dry yeast cells are is healthier after rehydration than liquid yeast cells that are propagated the way that most home brewers propagate yeast.
Dry yeast is propagated aerobically below the Crabtree threshold in a bioreactor (propagating below the Crabtree threshold results in the carbon source being consumed via the respirative metabolic pathway). Aerobic propagation produces yeast cells with fully-charged ergosterol and unsaturated fatty acid (UFA) reserves. The reason why we add O2
to a batch of wort is so that the cells that have been pitched can rebuild their ergosterol and UFA reserves. Ergosterol and UFAs make yeast cell membranes more pliable, which, in turn, makes it easier for yeast cells to pass nutrients and waste products through the cell walls. Dry yeast cells can skip this step.
Liquid yeast is propagated above the Crabtree threshold; therefore, all reproduction is fermentative. Only the initial mother cells have fully-charged ergosterol and UFA reserves in fermentative reproduction. These mother cells share their ergosterol and UFA reserves with their daughters during each budding event. The reason why we pitch at high krausen instead of allowing a starter to ferment out is because all replication after high krausen has been reached is for replacement only, which means that we are wasting ergosterol and UFA reserves. In essence, the health of a culture decreases as a fermentation proceeds towards quiescence.