Are my head studs stretching? KZ305

I'm putting the head on my 82 KZ305. Following the toque pattern for the eight head nuts, I get them all tightened down to the recommended 18 ft-lbs. Then when I re-check them, following the same pattern, they start moving at around 14 ft-lbs and turn about 30 more degrees untill 18 ft-lbs is reached. I repeated this 4 times. Could the head and base gasket be compressing farther, or are the studs stretching?

These aren't stretch bolts (hence the need to "re-torque") I would be a little cautious that the studs aren't pulling out of the crankcase like "corks from a wine bottle" (I have had this with a CB1100 !!)