AusDesi ji
Part of the variation lies in differences in the way beards grow. The distribution of facial hair on individual faces can vary greatly, giving more or thicker growth on the sides from the sideburns, to more under the chin, to more centrally on the chin. This is genetic, and reflects patterns of hair growth that are probably genetic.
No cutting is the bottom line. No dyeing of hair. Some even say no threaded or braided hair -- I think that is mainly an AKJ rule and have not read it in the Sikh Rehat Maryada. But even with no cutting there are differences in the way the beard grows in.
Differences in appearance can be related to how a beard is groomed. Some Sikhs will tie their beard with a hair net, some use gel and tuck it under and pin it in place, and some let it flow freely. Others comb through the beard with grooming products or even cocoa butter, giving a tamer appearance. That accounts other aspects of variation.
An analogy would be identical twins with great uncut mustaches. Both have identical patterns of hair growth, and equal length. One twin waxes his to a point, and the other twin does nothing and the mustache is bushy. Twin number 1 has a handlebar mustache; twin number 2 has a walrus style mustache. So they are structurally similar mustaches, but they look different. The appearance is different in spite of no cutting.
If I am not comprehending your question, then you can check the Sikh Rehat Maryada to get the basic requirements.