unusual head torque issue

[Great Lakes Greg]

Not really an issue, just curious. I have a fresh rebuild with a 5.5 head initially torqued to 60 lbs. Ran it once and got some good heat through it, cooled off completely and went to re torque the nuts. Unlike a dozen times before on various engines, the nuts held they're torque. I pulled up to 75 pounds thinking they would "TINK" and then turn at a lower value, but they didn't budge, so I let off. I'm fine with it, I guess, just never experienced this before. Thoughts from the experts?