Why does a customers first payment amount differ from the rest?

In an ideal world, all plans would be created so that they were equally divisible by the number of months. For instance, a £100 over 10 months would be £10/month.

Realistically, to make the platform as flexible as possible allowing you to create bespoke packages for your customers this isn't always the case.

Take the following example;
A plan for £299 over 12 months would be £24.9166666667 which is impractical. If we round it up to £24.92/month the total would be £299.04 and if we round it down to £24.91/month the total would be £298.92 and neither of these values match the service plan estimate of £299 which you provided the customer.

To combat this, we round the monthly figures down and then add any remaining to the first payment. So in this instance £24.91 x 11 months = £274.01 so the remaining 8p is added onto the first month making it £24.99 making the total amount collected £299.

So:
1st payment of £24.99
11 payments of £24.91
Total £299