I also think of the first steep as longer because the leaves are hydrating first, then starting to release flavor into the liquid. In the second steep, the leaves have been hydrated, and while sitting wet, some of the goodies have already diffused out from the interior of the leaves to the water on their surfaces, and that just needs a flash rinse to recover those goodies in the second steep. After that, the remaining steeps need more time to get at the stuff that is held more tightly inside the leaf, which needs time to find its way out of the leaf to the liquid.
That's my just-so story at any rate, for why I do longer first steep, short second steep, and longer third steeps.
I start my senchas about 1g/oz of tap water at 160 degrees, 30", 10", 30", 45", 1-2 minutes, 2-3 minutes. If the tea seems to be losing a little sweetness, I up the temp to 170 or even 180 in the later infusions; if the first infusion is more astringent or grassier than I want, I might drop the temp, starting as low as 140 degrees. And some days, when I run out of time or stomach capacity too quickly, I fill the kyusu with cold water and put it in the fridge for a refreshing evening cup of cold-brewed sencha. And if dropping temps, I usually extend the time a fair bit too.