When he's reaching for the shelf, her height is such the top of her head barely reaches his chest and his eyes are about in line with the shelf.
On the next page, the top of her head is in line with his nose.
She looked WAY too short when he was reaching, yet in the panels before that, it looks like the shelf height compared to her is consistent with the shelf height when he was reaching. Checking previous chapters, her height seems to be consistent with the top of her head being parallel to his nose. As such: the mangaka really wanted that shelf height trope.