My two bobs worth.
The AAP prepost has a book of 150% plus. At jump the book is nominally 100% less the rake. In order to do this the prepost prices must as a whole lenghten to achieve the 100% (83% tab) (Betfair has commission out after the event).
Only those with the best chance will buck the lengthening trend in which case those lengthening will have to lengthen further. Herein lies the dilemna- how to pick the shortening ones and get better odds before they shorten to the point of negative return.