While we promote prices for fixed periods, like 7 and 14 nights we do allow guests to add extra nights to their bookings.
For instance guests may want a 9 night booking instead of just 7 nights. In these circumstances we add the extra nights at an average rate based on the 'next shortest' period you've already given us.
For example. A guest wants 9 nights. You've already given us rates of say, $280 for 7 nights and $420 for 14 nights. As 9 is less than 14 we use the next shortest period, which is 7 nights. The 7 night average 'nightly' price is $40 a night ($280 / 7 = $40) and apply it for the whole 9 night booking. This means a 9 night booking would earn you 9 x $40 = $360.
You may find guests in this situation my actually book and pay for 14 nights and leave early. This increases their flexibility and earns you a little more money.