buildworld in current vs stable/14

I am not sure when this changed, but I often test building the system 
just to do it, often with ccache. What I am seeing is that in 14.x 
systems, ccache shows a total number of ccache hits and misses in the 
low 40k range, whereas in 15 it is much less, often 2-3k. Of course the 
build moves more quickly. Is this tech to only build what is related to 
code changes?

Brian