Came to me as a surprise as well.After reading a few posts on google and bitcointalk, I finally found something useful.Although it doesn't seem relevant to the current tread,it makes sense somehow.Check the quoted post below.
The client calculates the priority of the transaction, which is defined as coin days destroyed (i.e. number of coins times coin age) divided by transaction size. If it is above a certain level, the transaction may be sent for free. Otherwise, a 0.001 LTC per KB fee is enforced by the client. In general, this also happens to be the minimum recommended fee per KB.