I think it's probably pretty hard to be as accurate as she is. You can try it out with a rubber band anchored at one end and the other in your hand and stretch it to increase and decrease tension while strumming it in the same way and see how many notes you get dead on when you try to play a song. Of course, shaking it to get vibrato kind of helps put you in the range of the note so you can make small adjustments without being too obvious. But still, she's impressively accurate, and this clearly sounds way better than your rubber band will.

Both beats are minor variations of a standard guitar strumming pattern so it's not at all surprising you can make them fit perfectly together. Mainstream music uses just a few standard beat/chord patterns, and you can find many more examples of such "stealing", like
http://www.videosift.com/video/A-Brief-History-of-Pop-Music-in-Four-Chords and
http://www.videosift.com/video/The-Pachelbel-Rant-Never-look-at-the-Canon-in-D-the-same

Steve Goodman died from leukemia in 1984. He was 36 years old. He was one of the greatest flatpickers to ever strum the strings. He also wrote the City of New Orleans as well as some other amazing songs.

This song is self explanatory.

I've listened to it a hundred times, and I've found two mistakes. One is around the 5:00 mark. I can't remember where the other one is. On YouTube, the commentary agrees that her strumming technique needed improvement, but that in later recordings as an adult (this was about ten years ago) her technique was much improved. I know diddly about classical guitar, so I can't comment.