Did you not read the above article that was posted earlier?
http://nces.ed.gov/nationsreportcard/pubs/studies/2006461.asp
It's one of dozens that say the same thing - private wins over public any day of the week.
Why - Perhaps the parents are doling out some serious $$ and they want to make sure their kid is earning that education. They take a more active, more positive role in the entire process.
or
since the class sizes are smaller, the teachers are better equipped to identify and mentor struggling students.
or
Teachers don't have to 'teach to the test' (SOL's) so they have the flexibility to delve into more interesting, more stimulating subject matter.
After all, students aren't motivated to learn if they could care less about what's being taught. Piquing their interest is key.