I don`t think one answer would fit, sometimes covers it best. The water here in Ohio and up there really doesn`t get warm enough to be outside a bass`s normal comfort range. I have caught L/M 20 feet down in September on the Georgian, after getting up to ice on the edges of the bait bucket. Also got them in 2 feet at other times mid-day in the summer, same here.
Water temp was still in the comfort zone for them, all I can see as a reason is food supply, bait fish moved, they followed. Just my opinion, you limit yourself if you only fish where you expect to find them. You need to be able to cover a variety of depths, bass can be unreliable.
At night I usually fish shallow, the edges, less than 10 feet. Just easier to stay on targets though. Daytime I will hit areas shallow and deep. Smallies on Erie mid-summer I never wasted time shallow 15-25 first, then deeper.