What I see (IMHO) is
porters between 2003-2007 follow a guassian pattern with peak centered around 2005 and taper off at the edges. Hence 2006-end to 2007 who interfile now (between May 2012-June 2013) would be quite a few compared to porters who interfile during last 6 months in the region of 2004-2005.
FYI- 600 per month interfile every month(only pre-july 2007 Eb3). if 350-400 lie before 2005 and part of them get greened and keep the PD at 2004 SEP and leave the balance added to demand.
Balance 200-300 get added every month to demand and hence rise in demand numbers.
Just my thoughts only