Maxperson
Morkus from Orkus
Flammable means can be easily set on fire. I use the match test -- if a lit match touches the object, does it catch fire? Not held onto, but touches. If so, the item catches fire. Oil is flammable. Paper is flammable. Thatch roofs are generally not flammable (despite what movies show -- people tend not to make their houses firetraps). Wooden doors are not flammable. Curtains may be flammable.
This makes total sense if the amount of initial fire damage is the equivalent of a match. If it's doing say 12 points of fire damage to the target, though, it's a hell of a lot hotter than a match and will catch correspondingly more items on fire in that instant.