The wheel scrolls the Dashboard vertically, because that is the most often used scroll operation in the Dashboard. No extra keys needed.
Alt+wheel does nothing (same as wheel). Shift+wheel scrolls horizontally.
In the timeline the most often used scroll direction is horizontally (along the time axis), so this is what the wheel does without any additional key.
In the rare circumstance that you have so many overlapping events that you actually need to scroll vertically to see them all, you use Shift+wheel. Alt does nothing either.
Ctrl+Wheel zooms in and out in the timeline and the dashboard.
That's what the primary wheel action is based on - the most frequently used scroll direction. Which differs between these features.