Why is the initial month of credits less than expected or the 2nd month unexpectedly more than the first?
Please see the below description from our Credit Consumption Documentation in the section: Initial month.
We will adjust the use of Fivetran Service Consumption Table by pro-rating your first month’s purchase so you’re not penalized for starting the service mid-month. This helps right-size all of our new customers and ensures that you can start Fivetran easily, at any point in the month, without wasting Credits.
For example, if your subscription starts on the 15th and it's a 30-day month, then we will adjust the Credit Consumption rates in the Service Consumption Table by 0.5. The pro-rating will take into account how many days in the month have already passed and the number of actual days in that month. This will be determined by Fivetran in its discretion.