So I asked Mr. Fiance about this, since he’s the super Star Wars nerd. Luke and Leia, being twins, do have the same midichlorian count. But Luke, having been trained, is able to see the visions of Yoda, Anakin and Obi-Wan. His actual words: “You can have all the midichlorians you want, but if you’re not trained, you won’t see anything.”
Something to consider is the fact that it’s impressive for Yoda and Obi-Wan to have been able to appear that way in the first place. It’s not something every Jedi can do. At one point in Episode II, when Anakin goes to kill the Sand People, you hear Qui-Gon say “Anakin, no!” Anakin doesn’t hear this, since he’s angry at the time (and anger/fear/hate is a path to the Dark Side), but Yoda does.
In Episode III, at the end, when Obi-Wan is going to Tatooine to watch over Luke, Yoda says that he has training for him while he’s out there. That’s the projection thing he’ll be working on – Qui-Gon apparently was at least able to project his voice, if not his image.
So: Yoda learned it from Qui-Gon, and taught it to Obi-Wan. As for why Anakin is there, we can either assume it’s because he’s the most powerful Jedi ever, or we can just accept that Lucas stuck him in too in order to play on the fact that he repented before he died.
Booya.