Hi:
It's almost like asking why sh#t happens...it just does.
Apparantly , things like internal spring tension ( and changes with time and wear) and small ball detents that are part of the internal workings...all conspire to make them lose calibration. Some manufacturers require you to "unload" the spring tension after every use, and not store it in its tensioned condition.
It is very common in industry to have these periodically re-calibrated, with a certified date / calibration sticker attached...to make sure all is still OK. The beam type, OTOH...is simply a long steel "beam" that bends and returns to position..and unless it was dropped and the initial setting is no longer "zero"...it is usually very accurate throughout its service life. It would work as a very cheap tool ( usu. $30 or so)... to "check" your click type...IMHO.
-- Wil Ferch
