I have a much loved beautiful English Staffordshire bull terrier.
She is beautiful proportioned and 100% healthy.
I've only owned staffys in the past and she looks exactly like my last 3. how ever this one is 8months old height at shoulders give or take 43cm and her weight 20.9 kg
She is a bullet.
She has never been through the leggy stage, has been like of solid looking build since pup.
Is this possible? Or has somewhere along the lines been altered with another breed?
She is much apart of us as my children and loved. So this is not about fear or disappointment it's just for my own curiosity to know.

Is she registered?

So she is currently 2cm over the standard desirable height range. There will always be some dogs who are a bit over or under size even with those who breed to the standard. If she is from pet breeding rather than show there can be alot more variation in size and type.
You say she is 20kg, what is her body condition like?