That is true. Few spots is not a sign of low sales in each circumstance. Some stations had a policy of very low commercial loads as David described.
I knew of a few stations where that was a case. In Sherman/Denison, Texas, the AM got almost all the money and the automated top notch beautiful music FM sister had barely two units an hour on weekdays and perhaps one an hour at other times. I recall in Llano, Texas, the first station in this small town was beautiful music (a strange choice for a ranching community) and if they had one spot an hour it was on weekdays 6am to 6pm. This was one of those stations that did 3 newscasts a day and only weather at the top of the hour, so if you liked 60 minute music hours, it was a good listen. They arrived on the scene around 1980. KBUX, Quartzsite, Arizona, when the Burdett's had the station, it was primarily beautiful music with some lite pop hits tossed in. Outside the winter visitor months when they had 10-15 advertisers, they were commercial free. All their billing came in a few months every year. They didn't do news or weather either, so all you got was a quick liner every 15 minutes and wall to wall music. I don't think they ever did more than break even in some years. You can Google them. They ran from a home in Quartzsite.