On busier flights, this is normal. The front row of PE isn't available in advance, so usually your seating will involve that row. The seats are held back, as per the reasons already given in response, so any lack of seats at the moment should be addressed when they are allocated.

Not to say that it couldn't be oversold, especially if there has been a change of plane since booking, but most likely it will be sorted when that row of seats are included.