Check frequency does not equal log frequency

Check frequency does not equal log frequency

We have a website monitor set to a check frequency of 5 minutes. But in the log report, the entries are 15 minutes apart. 

It looks like Site24x7 is checking the site every 15 minutes instead of every 5 minutes, as we frequently get alerted by our customers that the site is down, instead of by Site24x7. 

Any ideas why this would be?