Sensitivity means: they produce more volume for less power. Apparently, this can only happen with lighter cones. Heavier cones apparently are more neutral (or less colored or whatever term you ant o use). I think this is because heavier cones are less prone to "breakup" at higher frequencies.

This is my best understanding of WHY sensitive speakers are prone to more coloration (or less neutrality).

Comments welcome!
thanx
-akhilesh