Southern's press release claimed that leaves not only fell too fast but that they were of the wrong consistency when they landed on the tracks, giving a Teflon-type coating to the metals, which accordingly saw their punctuality nosedive from that reported during the same 'leaf fall' period of 2009.
I full understand the problems train operators and Network Rail face from leaf fall and am categorical that if you took a disheartened, unsympathetic Brighton-London commuter, sat him down and demonstrated the issues leaf mulch on the line causes in terms of reduce adhesion and circuitry problems in identifying a train's location, he/she would eventually be won over. Getting the same recent leaf-fall convert to then come to terms with what is effectively the "wrong type of leaf" would undo all your hard work.
It might technically be true and it might technically have accounted for the delays but it is not what people want to be told in a manner which appears to seek solely to justify the reduced punctuality of a train operator. It has echoes of British Rail's now infamous "wrong type of snow" press release that gave newspapers days of fun.
Taking a more cynical tack, the manner in which a train's punctuality is recorded requires wholesale alteration. If a long-distance service was checked at every station to ensure it departs within 9:59 (4:59 for local services) of its timetabled timing, rather than only the route's terminus being the only station that counts, the emphasis Southern et al place on a slight decline in their 'leaf fall' period's punctuality would be nothing compared to the hard work, in conjunction with Network Rail, required to ensure services are consistantly reliable and operate as per the advertised times.