Lower velocity with tight 6XC neck bushing ?

[eggman]

I am fine tuning a load for a new 6XC barrel adjusting seating depth, neck tension etc.. When I went to the tighter of the two neck bushings that came with SSS's 6XC die set my velocity dropped 100 fps with no other changes since chronographing the load with the larger diameter bushing. I would have thought that the velocity would have increased with tighter neck tension. How would this decrease velocity? My groups did get tighter and more consistant.
I am using 107 gr. SMK's, N-160 and Fed. 210 match primers with Nosler brass un turned. I was getting 3055 fps with 38.7 grs. n-160 and now I am getting 2955 fps. across the same chronograph.

Thanks,
Rodney