I think the general (completely speculative and unconfirmed) consensus is that "Turbo" models are somehow quantized, or otherwise modified for much faster and cheaper inference, at the expense of some quality (the definition of "some" is unknown here).

well, that's how it was for GPT-3.5 anyway. The "turbo" flavor was faster and cheaper, but seemed to have slightly worse output (again, this is all going by subjective measurement; it could entirely be the imagination of AI bros)