mobs take fall damage on a lead

wile flying with a mob on a lead if you slowly descend to the ground the mob will take damage as if it fell which dose no make sense because the mob is being held by a lead and is not falling .steps to reproduce:
1:fly up slowly with a mob on a lead so the lead dose not break
2:fly down slowly with mod on lead so lead dose not break
3:the mobs will die or take fall damage if you flew high anough