Not altogether too surprising. All the "standard" candles have some uniformity issues.Subtle differences in metallicities affecting P-L slopes. Doesn't look like it will be earth shattering, though, just some tweaking of parameters.
Like supernovae, this is not necessarily a homogeneous population of objects, but rather a population of similar objects. I appreciate Sandage & Tammann's candor though. Nice work by a couple of pros.
Pete.
edit: on second thought, if it weakens the distance scale....the need for dark matter might soften/or strengthen quite a bit. This might be much more interesting than my cursory inspection. I'll leave it to the distance-scale pros to sort it out though.