As an engine runs, its valve train is subject to some wear of cam lobes and tappets—wear that tends to increase cold valve clearance. At the same time, so-called valve seat recession also takes place. When the valves are on their seats, some local micro-welding of valve to seat takes place. When the valve lifts, the micro-welds are broken, creating wear particles. This loss of material causes the valve to ever-so-slowly seat more deeply, decreasing cold valve clearance. Valve train and seat wear can occur more quickly with use of aggressive cam profiles, which typically require stiffer springs, causing cam and tappet to wear from extra seat pressure. Same for seats, when higher rate springs shut valves harder. Ideally, the two gradual effects would pretty much cancel, resulting in constant clearance. If there's more money in the budget to assure this desirable outcome on premium models than on lower-markup bikes, we can just chalk that up to a free market economy.