Through her training she has developed:
a) the specific neural pathways to do that pose
b) the strength to do that pose
So why can't you do that?
a) you have never trained to do so and lack the "muscle memory" so to speak, even if you are otherwise strong enough
b) she's a skinny female and the absolute strength required for her to do it is less than required for you to do it (I'm assuming you weigh more than her). Smaller people tend to be stronger relative to their bodyweight. How many 250 pound bodybuilders do you see doing one arm pullups?
Strength is only part of the equation. Neural adaptation is another part. Let me give you another example. Sometimes people who have a good base of strength will try something new, and even though they have the strength to do it, they can't do it very well. This often manifests as shaking through the movement. Take someone who has been bench pressing for years and is pretty strong, but all they've ever done is bench press. Now have them do ring pushups (
image); they will shake like crazy!
Why? Is it because they're not strong enough? Of course not. It's because it's a new movement and their body is going "whoa, what are you making me do?" New neural pathways are being used and the body is not yet efficient at them.
Sometimes people mistake this occurrence as "he's not strong enough." A more accurate statement would be "he isn't yet efficient enough" or "he isn't yet coordinated enough to perform that new movement effectively."