The fading triceps was seen in Sergio Oliva, Chris Dickerson, Dorian Yates, Ron Coleman, Kamali, and many more champs. Not sure what causes it but it is a real phenomenon. The gap between the elbow and triceps seems to grow wider with age or time training. Could be related to injuries. The elbows are easy to damage.
Don't put your elbows on any pads while training. This includes biceps and triceps machines. It is too easy to damage the sheath and connective tissue there. All my latest machines have the pads cut short of the elbows so no contract with the sensitive area occurs.