Kanthal wire coils "wear out". As they are used, dry burned, rewicked etc etc... the aluminum oxide (alumina) component - which provides a self-generating, protective oxide layer that is both thermally conductive and electrically resistive - degrades, leaving primarily iron and chromium. Once you're down to that, corrosion will set in, and for many, you can "taste" the iron.
Resistance can, oddly enough, stay near the same... resistance increases due to loss of conductivity, while at the same time, resistance decreases as the alumina insulation disappears. What you see on an Ohm meter can often result in a wash... although very few people allow a coil to degrade that far before replacement.
Under a high power loupe, you can see the alumina start to "pit"... then you start to see patches of iron... then pretty much all black, and no more aluminum. If you've gone that far... you're really milking a coil for all it's worth. ;-)