When you complete the purchase of a property, the completion date is typically in the middle or towards the end of the month. The interest on your mortgage loan begins to accrue from the completion date until the end of that month. This accrued interest is usually added to your first mortgage payment, which is normally taken the month after completion.
An example
Let's say you complete on the purchase of your property on 15 June. The interest on your mortgage loan accrues from 15 June until 30 June. Your first mortgage payment, due in July, will then include the interest accrued during that half-month period in June, as well as the regular monthly payment for July.
The result is a higher initial payment, because it covers the interest for the period between the completion date and the first regular payment date.
It's worth noting that this initial interest figure depends on the size of your loan, the interest rate, and the number of days between the completion date and the end of the month. From the second month onwards, your payments will follow the regular schedule set out in your mortgage offer.