From the Wikipedia entry on "Jaffa Cakes":
http://en.wikipedia.org/wiki/Jaffa_Cakes
McVities defended its classification of Jaffa Cakes as cakes at a VAT tribunal in 1991, against the ruling that Jaffa Cakes were biscuits ... The product was assessed on the following criteria: ... The product hardens when stale, in the manner of a cake.
The implication here is that while some things (e.g. cakes) harden when stale (presumably by losing moisture) while other things soften (e.g. biscuits) when stale (presumably by gaining moisture).
If the chips got directly rained on, then that would be "soggy", but if you left the bag open on a humid day, causing them to get a bit damp, that would be "stale".