The rowdies were made with entirely different materials.
Instead of sugar, spice, and everything nice, they were made with snips, snails, and puppy dog tails.
Buttercup's base composition is fundamentally different than theirs; she is a "perfect little girl", they are "perfect little boys", per the nursery rhyme.
The professor is very interested in science; aside from the girls, he has created:>a time machine>a vaccine>turned apples into oranges (sort of)>made people switch bodies>A giant uncontrollable robotic death machine>a battle suit as powerful as any of the girls>A "cure" for chemical X (see, he is interested in their biology>A sentient lifeform called Beebo>Basically Knight Rider
Also he is apparently a certified optometrist
And these are just off the top of my head