Here's why:
* Flip Summon: A flip summon occurs when a monster that is face-down in defense position is flipped face-up by an effect (usually its own) and summoned to the field.
* Attacking Facedown: A monster can attack directly (without attacking another monster) if it meets certain conditions. However, this does not change its face-down position, nor does it summon it in any way.
In short:
* Flip Summon: Changes a monster's position from face-down to face-up and summons it.
* Attacking Facedown: A monster attacks directly in its current face-down position, without changing its position or being summoned.
Let me know if you have any other questions!