The reasoning behind the decimals is to account for instances that ran for only a portion of a day.
The billing file is used to calculate this metric. The CloudHealth platform will look at the number of running instances per hour of each instance type and that result may include partial instance counts if they did not run for a full day.
A basic example of this calculation: If you ran one instance for only 12 hours of the day, the daily total would 1 instance x 0.5 days (24 hours/12 hours = 0.5) with a result of 0.5 instances run. Every calculation is taking into account the full potential value of the time interval.
This can not be changed to whole numbers but should provide an accurate reflection of total instance counts based on run time.