It simple mechanics boy's, loose tolerances, worn pads, possible air-bubbles and heat in the system. If you race or ride hard on the street your are going to have to keep all tolerances as close to "new" as possible. New pads in the calipers reduces volume in the piston bore minimizing travel there for pressure on the pads and reducing travel at the MC respectively. OEM MC's are not constructed to the same tolerances as forged, close-tolerance, superior sealing gasket compounds of high-dollar aftermarket race components. Air in the system causes problems but careful attention in bleeding learned from experience is the best cure there. If you really want to ride fast and safely,
you are going to have to keep things as "new" as possible! Just go ask a professional racing team, they mostly use broke in components within 99% of being new in matched sets and in most cases are changed after each race. That's my two cents here!