It's more complicated than one neurotransmitter == 1 float. For instance you can have more than one receptor type for each neurotransmitter. And each one might be sensitive to different concentrations of that neurotransmitter, and have different signaling modalities.

And the behavior of those receptors might also be modulated by other signaling mechanisms inside the cell: i.e. neurotransmitter A might modulate the signaling of neurotransmitter B.

In other words, there is a lot of complex information processing going on there.