I think that Alan hit the very head of the nail, and his comment rings true re. not only kerises, but ethnographic arms ( and not only arms :-)) in general.
Somehow, we all have iron-clad patterns, or "geshtalts", of what constitutes a particular type of weapon, be it its ethnic or tribal origin, defined name ( G.C. Stone, thank you! :-) ), age characteristics etc. We get apprehensive when we encounter something that does not seem to fit the agreed-upon pattern. This often leads us to either pronouncing the item a fake, or to our exultation of discovering a new and hitherto unknown example of native weaponry.
Ih fact, and that's exactly what Alan hinted at, any old weapons were not mass-produced according to some government-dictated standards, but were hand-made according to the professional level or inventiveness of the master, availability of the materials, wealth, desire and taste of the owner etc.
The variability is inevitable. It is seen in each and every culture, from Indian tulwars and their handles to Caucasian kindjals. Here an example from this Forum: a combination of a talibon-like blade and a gunong handle. Should we start looking for some deep meanings or just accept the fact that it is a very usable implement fully satisfying its owner's wishes, and no more? Occam's razor, anyone?
http://www.vikingsword.com/vb/showthread.php?t=17394