All internal flows have a characteristic variable called the Hydrodynamic entrance length. It is defined as the length from the entrance of a pipe (internal flow) required to achieve a fully-developed flow profile. For laminar flows in circular tubes, the fully developed flow profile is parabolic. For turbulent flows, this is flatter due to turbulent mixing in the radial direction.
Fully developed pipe flow profiles are valid only after the flow becomes fully-developed. Since, you are not considering the entrance length in your calculations, you are getting a wrong estimate of the losses.