I can only surmise that you have an account where stocks can be purchased and held in street name, meaning the broker buys it and holds it in their name. They can show this as allocation to your account because you have all rights to the stock at the time they purchase it.
Since the dividend amount has not yet been received by you, they must have a mechanism for charging you some interest for having used the broker's own money to buy the stock. Following that line of thought, I see nothing wrong with them pre-purchasing stock for you and then using that date as the date of purchase. If I were you, however, I would want to ask them about the interest or commission charge for this purchase.
