Maybe he kept his hands on your legs because he was still considering further adjustment of your position as he moved his hands up to knees. Maybe he was just tired and his back hurt, and he was propping himself up on your legs as he let go. (An ob/gyn nurse I know gets really exhausted yanking women's heavy bodies around. Her back hurts so much all the time that she has to go in for acupuncture just to function.)
Or, let's say you have totally gorgeous, firm, smooth supermodel legs and he was so impressed that he wanted to feel them. (Or maybe he's a sex maniac that would grab anyone's legs.) Even then, it's hard to believe he'd drop his mindset of mentally prepping for the procedure he was about to do and cop a feel. Usually, something sexy doesn't pop into your mind when you're focusing on doing a task. It's also kind of hard to credit he'd do such a thing where other people could easily see him do it. I would suggest that isn't what was happening. Likely he wasn't thinking at all about your legs except as handles to get you into the right position quickly.