3 different ways to write an equation and only one works, why?

In a glsl shader code for explicit time integration for the euler equations the equation

Fx=(cxl*ulf-cxr*urf)/dx;

occurs. There cxl and crx are both vec4 and ulf, urf and dx are floats. The test example is a rising heat bubble. Now there are at least four ways to write the equation above:

1. Fx=(cxl*ulf-cxr*urf)/dx;

2. Fx=cxl*(ulf/dx)-cxr*(urf/dx);

3. Fx=(cxl/dx)*ulf-(cxr/dx)*urf;

4. Fx=cxl*ulf/dx-cxr*urf/dx;

only the last one (4.) gives the correct result and the bubble stays symmetric, all the others make unsymetric errors.

anyone has an idea why that?

thanks

Stefan