charters

How do charter schools "steal" or "take" money from public schools? Please enlighten me. At School X, a public school of 1000 students, 100 leave for Charter Y. So yes, School X faces to lose 10% of their ADA (attendance funds) and other funds for sure, since funding is based primarily on the number of students.
But, let's say instead, School X loses 100 students to Private School Z, or those 100 students move all over and thus out of School District X's boundaries.
With the only logic I can figure about charters taking money- because they "take" the students that "belong" to school X, then can't that same logic be applied to Private School Z or the students moving? So we can just as easily blame private schools and all the factors behind families moving.

Also, do the public schools expect 100 students to leave to Charter Y and yet retain the funding? Because again that does not happen when they go to a private school or move.

So it is either a ludicrous argument or I'm missing out on some important detail. Please let me know.