[Jac Mac]

Alloy expands at a greater rate than steel etc, in a pushrod motor the block/head will expand/grow while the pushrod stays much same creating most of the extra clearance, Plus the cyl head alone grows more than the length of the valve stem to create a bit more & in the case of the exh valve head dia growth also contributes slightly [ that valve head will also do the same on an iron head as well though, so not really relevant to this discussion ].