Experience, practice, training and time devoted since one became a guardian, then also each individual natural abilities about learning are different and vary from one guardian to another. Also being in the golden age might have factor in too. Becoming a new light after the collapse might limited new guardian to study and choose one class/sub class or being chose for by the vanguard trainer in the tower after basic test and exam. As they grow stronger in light "power" more paths are open up, take for example the first Destiny in game, guardian are able to control a second sub element after reaching a certain level. Then when we are much stronger, we obtain the third element in the Taken King and seems more like the method was being able to absorb those new element power (if I remember correctly). Sunbreaker got invited and pass a trial then obtain the power from a relic, Warlock being told (taught) be Ikora about Stormcaller and how to harness its power. The most shocking and bizarre was when Cayde told us to kinda steel the power from Trevor before it fades away and in the dialogue Cayde said that Hunter that could uses void were really rare. So to summarize from my personal understanding, First being more experience in knowledge, battle, training and study or research the light as one grew stronger are able to tap into more different path of the power in the light. Imagine some of the first wave of guardian could be hundreds of years old. Second, the method of obtaining the element or class varies, one could study and learn from nature, being taught by another, absorb from relics or artifacts, even steels from or pass on from another guardian. On the last note, was if "light" power in Destiny universe are the same as light in reality? What if the power lays in its spectrum? We seen three colours, what if they were more that we have yet to witness just like those wavelength that spread when the light passes through a prism?