Johan is right. The problem is, the mail retrieval process is a "dumb" process -- it gets called on cue every x number of minutes irregardless of whether the previous process has finished or not or whether or not another fetchmail process is already running. If you set the interval too low, you will get a lot of notifications about another fetchmail process running ....
I remember while still using Windows based mail servers, some actually will allow an interval of 1 minute but that one minute countdown does not start till the previous mail retrieval finishes or times out due to error(s).
Kelvin