I asked a question. How is that "not how statistics work".
Did you really think I believed that since I passed 512 eggs I was entitled to a shiny? Do you really think I don't know how probability and statistics work?
That's kind of what it looked like. In that case then, I think that 1/512 chance is for chaining SOS encounters, and some comments I've come across say that it's probably similar to that with breeding. I can't find anywhere that has a solid answer.
Well to answer your uncertainty about the sos chaining, it's actually 1/683 on a chain of 70 or higher (except after 255 when it goes back to normal). Can't find any info on masuda method though.