When you say "cheeky" are you referring to physical characteristics or attitude?
And are we talking about some specific cheeky girls - in which case which ones - or cheeky girls in general?
I have been doing a topic about Variation in science at school. The reasons there are some differences are mostly environmental.
If Monica is taller, it might mean that she ate healthier stuffs than Gabriella. So she might be taller for that reason. One of them might be a little fatter than the other, thatís because she doesnít eat as much healthy stuffs. I also noticed that they wear a lot of makeup. The shades you do with the makeup might make their faces look different. They have a lot of money, so they mightíve used that money for surgery. Also some people prefer having their hair on the side and others in the middle, which might be why they look a little different. This also means that they have their own style of eyebrow.
Also, when they were born they looked very alike, but when twins grow up, they look different because they donít have the same DNA; nobody in this world has the same DNA as someone else, not even twins. So they might have the same genes, but never the same DNA. Maybe they had accidents when they were young that made them look slightly different. So scars could be a reason. Like maybe when they were in the belly of the mom