ACCRINT()

when i use the function ACCRINT(),I meet a problem.
ACCRINT("39507","39691","39569",0.1,1000,2,2)=17.222222222
BUT:
ACCRINT("39507","39691","39569",0.1,1000,4,2)=16.666666667
39507---"2008-2-29"
39691---"2008-8-31"
39569---"2008-5-1"
As you see, when "Frequency"=4 the result is smaller.
who can tell me why?

Thank you very much!