Drivers&mechanisms:convergent forelimb reduction in non-avian theropod dinosaurs

I am not a biologist, I don't get this.

They say some non-avian theropod dinosaurs convergently evolved smaller forelimbs.

But note the "some".

Aren't they just stating that the ones that the ones that evolved smaller forelimbs all evolved smaller forelimbs, and the ones that didn't, didn't ?