Literally every piece of the valvtrain flexs, stretchs, bends, something! From the hold downbolts, to the shafts, rockers flex, retainers flex, and all this can be seen in the design of very high end valvetrain components that are stronger to combat this. My Jesel setup with a gross lift of .867 lost .040 versus checking springs that checked spot on. Even the cam flexes, hence the bigger diameter core cams used on some race motors. Ater seeing that .040 loss of lift i decided to go with a lighter spring, taking about 50 lbs of seat load off the spring at rest. That showed less deflection of .030, a savings of .010 in net lift.