question on a frisian tactic

i was wondering about a solution and i'm not sure where to find the necessary resources to find out why haha

after 2. 26-21, 17x48, why does 3. 28-23 not work? i'm aware that there is literally no other moves but i'm wondering why black can't just play 18x27 and is instead forced to play 48x34?

thanks in advance.

beacuse of majoiry rule, black has to take 2 pieces