Because the Z axis movement is quantized by the stepper motor, the values returned by BL Touch cannot be continuous and are also quantized.
If you double your Z axis stepper microsteps the quantization you are seeing will reduce in half, etc.
EDIT: For example, my modified CR-10S Pro uses 4 mm lead screws with 0.9 degree steppers, which gives me a 0.01 mm resolution per full step. I therefore would get 0.000625 mm resolution per microstep with microsteps set to 16. In practice, I use 64 microsteps (because I can) which gives me Z resolution of 0.00015625 mm.