If kundalini is universal, why do some kundalini yogins seem to have more kundalini-energy than others?
It's an intriguing question. If an individual's kundalini is viewed as simply a personal reservoir of a cosmic energy then why would one person appear to have more of a reservoir of kundalini energy than another? Nevertheless, this does appear to be the case. This is probably another advantage of the viewpoint that prana (or qi) is the same as kundalini.
Some Chinese texts distinguish between "innate qi'' or "pre-natal qi'' that one is born with and "cultivated qi'' that can be developed. Clearly some people simply have more "innate qi.'' This manifests as a stronger more resilient body and greater general vitality.
Through training those that have relatively weak "innate qi'' may surpass those who have strong "innate qi'' but do not train. There are many stories in the Chinese literature of Qi Gong about people who took up Qi Gong in order to improve their poor health became powerful martial artists or great qi gong masters. Of course those that have strong "innate qi'' and also train their qi may develop the strongest qi of all.