Hi there,

It's possible that this person isn't playing the strum pattern the same as it was on the original recording of the song. Sometimes you may just feel the rhythm, and strum well-enough to make the song sound good. This doesn't necessarily mean you're strumming the way the original artist did it.

This is something you will simply be able to do through playing. Maybe that's not a very good answer, but if you let yourself have fun, and remove yourself, for just a moment, from the technical stuff, and feel the song, it may help you with your rhythm and your strumming.