Varying rates are also established by the packaging of the sales buy. If the advertiser is willing to accept a full 5:30A - 10A as their "morning drive", they might get a substantially lower rate than the advertiser who insists on only running from 8a-10a where the most listeners usually are.
If an advertiser is willing to advertise over a broad range of dayparts including nights and weekends, then the station can give them a break on their individual daypart rates by averaging things out.
There are advertisers that are around week-in, week-out, all year long. These guys are given substantially lower rates across the board at many stations since when stations get into a heavily sold week before a holiday, the year-long guys get shoved around into other weeks within the month. An example of a year-long advertiser is someone like the Phila. Ford Dealers Assn. They're around all the time, and I'll bet you can frequently hear their spots between 5:30 and 6 AM on many days.