Both my 09 YZ450F's were .001" too tight nearly all around after between 2-5 hours of operation. Checking them again after 15 hours, no change. It was only after 45 hours on both bikes that the clearances moved back into a wider spec.
Could be that of carbon build up. Though I'm not sure, only know that they were both too tight and needed slightly smaller shims at 15 hours. I then had to return them to the original shims after 45 hours.
BTW, my bikes have never shown any signs of wear on any of the journals, cam lobes, or buckets. Also I remove the head and jug every 30 hours to inspect and measure the rings always
reusing the original OEM
base, head and exhaust
crush gasket without ever having any issues. I currently have over 300 hours combined on both bikes.