I think the talk about differentiation is mostly an excuse not to do anything for the best students. Even if a teacher does differentiate well in a sequential subject such as math, there is usually nothing on the report card beyond an A grade in the subject. If an exceptional teacher manages to teach algebra to Jim in 4th grade while teaching the regular math curriculum to the other kids, when 5th grade comes all that Jim has officially done is pass 4th grade math. If his 5th grade teacher does not differentiate and build on his knowledge, his 4th grade differentiation may be mostly for naught.
To sum up, meaningful differentiation should be accompanied by documentation, and that documentation should determine what is studied next. The simplest way to do this is often to subject accelerate, but many educators are philosophically opposed to this.
