Cume is the number of people in a ratings period who listened at least once to a radio station for at least 5 minutes.
Take a look at the Los Angeles ratings. It’s stark, but far from unique:
http://www.radio-info.com/markets/los-angeles
While the ratings are going up for KFI (AM), their Cume is going down. The radio audience is polarizing into the people who listen to KFI (#1) and those who never listen to it. AM radio is the reason. KFI had a Cume of 1.2 million. KIIS-FM (Kiss FM) was #2, but had a Cume of 3.7 million. The people who listen to KFI listen 3 times as much on average, but only 1/3 as many people listen to KFI – ever.
The #2 AM station is KNX – the CBS All News station that has an almost identical Cume, but half the AQH (Average Quarter Hours). People listen to hear about traffic problems and the weather, then change back to something else.
You can’t directly derive this from the numbers, but it looks like only about 1 out of 5 people in Los Angeles ever turn on an AM radio at any time during the month. This is why you’re seeing the rush to move News/Talk stations to FM or at least FM Translators while it still has a chance to grab an audience. If 4/5 of the potential audience have never found the AM/FM button on their car radio, it doesn’t matter how good your programming is.