How quantization works

Assume a vertex has a position (x, y, z).
Does Draco quantize each coordinate x, y, z separately or does it combine (x, y, z) into a quantized value?
And how many quantized values should it be after quantization?
I checked my result, with qp=1, number of new vertices (let's call N) = 4
qp = 2, N=21
qp = 3, N = 217
qp = 4, N= 944
qp = 5, N = 1234

Should not the number of quantized vertices = 2^qp?