Fluid parcel + point of a fluid If we have a fluid (static or dynamic), and we pick a point in this fluid. This point is represented by a very small cube (or small volume). Inside this cube, it has the same properties (pressure, velocity, etc). 
But sometimes we take this parcel and we analyse the pressure difference between the surfaces of the parcel. This is confusing me, because a point of fluid has the same pressure and it represents a parcel. How come we can analyse a parcel the difference in pressure?
 A: You are mistaken in saying that a control volume has uniform pressure, velocity, etc. Usually we define an average pressure, velocity, etc. there and then it is very well possible that pressure, velocity is non-uniform in the control volume. We can then relate these to fluxes in/out at the surface of the control volume. 
Hopefully, this becomes more clear with an example from numerical analysis; consider the continuity equation which is valid at any point in the fluid including in our control volume:
$$\partial_t\rho + \partial_x \rho u = 0$$
Let's discretize this using Finite Volume discretization:


*

*We first define our average density in a control volume of volume $V_c=A\Delta x$:
$$\bar{\rho} = \frac{\int_{V_c}\rho dV}{\int_{V_c} dV} = \frac{1}{\Delta x}\int_{x_0}^{x_f}\rho dx$$
here, $x_0$ and $x_f=x_0+\Delta x$ indicate the location of the the surfaces of the control volume.

*We discretize the continuity equation along the control volume:
$$\frac{1}{\Delta x}\int_{x_0}^{x_f}\partial_t\rho dx+ \frac{1}{\Delta x}\int_{x_0}^{x_f}\partial_x \rho u dx= 0$$
which results in an ODE for the average density $\bar{\rho}$:
$$d_t\bar{\rho} + \frac{\left.\rho u\right|_{x_f}-\left.\rho u\right|_{x_0}}{\Delta x}= 0$$
we now have a relation between the average density of a control volume and the mass fluxes at the surfaces.

*A next step would be to discretize the time derivative (using e.g. a Crank-Nicholson scheme)


Now you might be wondering why we are interested in average quantities? We are not so much but if the control volumes are chosen small enough, the average densities start to approach the instantaneous densities.
