on newer engines especially. The heat/cool cycle causes expansion of the head which in turn may compress the head gasket to the point the gap is reduced. Conversely, in older engines, head bolts can stretch slightly causing a widening of the gap. Diesels especially, need to have the valves adjusted at scheduled intervals due to their higher compression, which exacerbates both scenarios. After rebuilding an engine I first run it up to temp, let it cool, and then re-torque the head, seems to help over the long run, but of course does not eliminate the need to "run" the valves now and then.