Check for updates frequency changes from site to site?

Hi Just found this plugin finding it really useful. I have noticed that on some sites it lets me choose a frequency of days,weeks,month and others it only allows me to choose seconds, daily, twice daily. wondering if anyone else has come across this and if so can you change it? I would like to have all my sites check once a week or once every month so daily is a little too short. Any help would be grateful.