Why does the interest added to my loan each month vary?

The interest on our loans is calculated daily, then added on to your loan each month. Because it's accrued daily it can vary how much is added depending on how many days there are in each month. If a month has fewer days (like February for example), less interest would be added, then in a month with more days, slightly more interest will be added because there are more days for it to accrue.

The interest is still decreasing after every payment you make, but simply varies slightly due to the number of days in each month.