WHY RADIO CUME IS VERY MISLEADING...

It doesn't take into account length of listening time.  From NTS MediaOnline:

Offering further clarification Leykis told NTS MediaOnline Today, “The cumulative audiences of terrestrial radio stations are not a count of unique listeners, but are actually a count of listening opportunities. For example, if you tune to KNX/Los Angeles every weekday morning to hear traffic at 7:25 and stay for a total of five minutes or more each time, you are counted as one* weekly listener in the station’s cume. In this respect, our cume is calculated based on the same principle as radio’s cume, in that our 401,000+ listeners is a number of ‘listening opportunities’ just as radio’s cume is, so we have a way of comparing ourselves to radio.”

UPDATE: The five minute rule as mentioned by Leykis seems to be accurate, at least for Arbitron:

Cume Persons

The total number of different persons who tune to a radio station during the course of a daypart for at least five minutes.