Just driven Sydney to Toowoomba and a bit of tripping around the Darling Downs area of QLD and checked out this issue over about the last 1500 or 2000 km or so.

I find if I let the overun get to about 8 kmh over the set (110) speed the inst fuel consumtion does rop back to near zero (0.1 l/100k).

It seems to me the CC is exhibiting proportional as opposed to on/off control, which keeps everything nice a smooth without jerks