Royalty amounts can differ from sale to sale due to the following three factors:
- Your royalty level
- The location of the buyer to determine the tax (VAT) rate
- If the buyer paid for some or all of the purchase with a voucher
To make it simple let's assume the buyer is in the UK and is subject to 20% VAT and you are a Bronze Author.
If the buyer pays in cash the 20% VAT rate equates to £0.17 VAT on a £1 purchase (£0.17 instead of £0.20 because prices must be VAT-inclusive by law). You would receive 70% of that amount, or £0.58. You can think of this as the minimum you will ever receive when the buyer is in the UK. Note that some countries have higher VAT rates, e.g. Hungary at 27%, where your minimum will be lower.
However some buyers will have voucher credit on their accounts due to TES promotions. This money is not subject to VAT so you'll get more royalty on these purchases. In the same example, if the UK buyer funded £0.40 of the purchase from a voucher and £0.60 from cash, 20% VAT would only apply to the £0.60. In this case VAT is £0.10, so your royalty would be 70% of £0.90, or £0.63.
You can see a breakdown for each of your sales in the 'Finances' tab of your Author Dashboard: https://www.tes.com/teaching-resources/dashboard/finances