I'd also say, as the answer below suggests, the "my" prefix comes from practice examples and homework and such. But some people might be using this in a very specific way. In the professional world, there's also a common practice to prefix class member fields with
m_ or _ - a form (and a relic) of at one time widely spread Hungarian notation. I'm guessing there are people who are using my as a variant of this (where, in methods, "my" indicates the object is referring to a variable it owns).