[atlas] one-off UDMs more expensive?

As I go through the web interface to setup a UDM I notice the following: For the same DNS measurement, when I specify an interval of 24 hours between samples, the total daily cost comes to 10. When however I specify this to be a one-off sample, the cost then doubles to 20. This doesn't seem to make sense to me. Is there a special reason for one-off measurements to be twice as expensive? ~paul