Phyto-Hormones are from plants that exhibit hormone-like properties. They will have a weak effect on hormone receptors.
Synthetic hormones are man-made chemicals that have hormone-like properties. They will have a strong effect on hormone receptors, even stronger than natural hormones. They will move into the hormone receptors and not move.
The analogy that I like to use is the receptor sites are like parking spots. Phyto-hormones will go into those parking spots and then leave and go into another parking spot. Synthetic hormones will go into those parking spots and not move.