Why would my girlfriend put her hand on my knee and rub it a bit instead of my leg?

We were cuddling and she tries something new for her which is good and I like when ever she touches me.

However she put her hand on my knee and soon after started lightly rubbing it. Why put her hand on my knee when I was wearing shorts and she could have more easily put her hand on my leg?

She is inexperienced so maybe she wanted somewhere easy and nonsexual to touch? Cause I have touched her knee before but I soon moved my hand up her leg slowly. She just stayed on my knee for a few minutes while we cuddled.
Why would my girlfriend put her hand on my knee and rub it a bit instead of my leg?
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