She's not sure why, but kissing him on the face had always seemed too intimate. Either she had to wrangle with a mask she didn't really like kissing (and made the mask too much of the center of attention) or she had to expect him to move it to make room for her, and even if it was normally innocent skin, the slightly lighter skin tone made it seem like it wasn't, and that was just too much fuss for just her.
So.
It made sense to her when she wanted to show her affection, that she'd lean up and kiss his adam's apple instead. That, at least, he left exposed for the world to see.
It seemed much less intimate and trusting than anything involving his mask, anyway.