Miranda is a 'biotic goddess' in human (without augmentation or resurrection etc) terms.
The idea is that she was both 'born this way' but also 'designed this way'.
She has some of the highest biotic skill potential of humanity. Like the top percentile, for sure.
But that doesn't necessarily mean that she IS the most powerful human biotic. Like anyone, she'd have to have focused training on it for many years.
Jack was a shortcut approach. A test to not just see if you could engineer not a better human, but the furthest extent that a human could reach without dying/going beyond insane.
Samara was a naturally gifted biotic (as most asari are) - who then takes the training of decades and centuries. If she was Miranda's age, she obviously also wouldn't be able to hold the biotic field.
Get Miranda to be closer to immortal and train her biotics for decades, and then we'll see how good she is. She has the potential; she is gifted.
Shepard can be explained as also gifted - but the mix of luck, dedication, and courage to act. This is why Miranda has a degree of jealousy of him. It is one thing to be born for something (which can foster all sorts of personality disorders), but another to witness someone climb to your level and beyond and also luck out not dying in the process.