This should be less pronounced since a patch earlier this week. But preferred travel times are essentially random, so all else being equal, the bookings should spread fairly regularly, with occasional outliers due to larger reservation sizes (these are randomised as well and in rare cases you might receive a “group reservation” for 15 pax at once).
You will eventually be able to achieve this using booking classes and fare rules. With these features in place, you could create a relatively small booking class that only gets matched by a fare that is a.) cheap and b.) requires advanced booking. And the opposite, of course. Once nesting is implemented, you could even ensure that there’s always more capacity for the expensive fare until the physical capacity is reached.