Summary:
Very rarely, but sometimes sunflowers can fall from double tall plants. It seems this is due to the fact that a second before breaking the plant, a sunflower appears (including the hitbox) and sometimes the player hits it "breaking a sunflower".

Steps to reproduce:

1. Place very many double tall plants (any except for sunflowers).
2. Break them all by hand.
3. If you have not received any sunflowers, repeat steps 1-2.

Observed results:
Do you get a sunflower from a non-sunflower.

Expected results:
Sunflowers will only drop from sunflowers.