because those are both isolation movements. isolation movements bring out the detail and striations in that muscle. when people go heavy on isolation movements, they sacrifice form and less tension is actually put on the muscle.
well say 80lbs was at 100% isolation, meaning all the tricep is invloved
you bump it up to 140 and now only 60% of your triceps being used, thats still 84lbs. Does that make sense? Idk if it does