Hi cef3rd,
The Crane paper #410 is the bible on fluid flow and pressure drop. I'd suggest getting a copy, here:
As to why there could be differences, take a look at various restrictions in this paper. For example, pressure loss due to pipe entrance versus exit geometry.
For a "well rounded" rounded entrance, the restriction coefficient, K = 0.04
For a rounded exit, K = 1.0
So depending on if fluid is going into or out of this entrance/exit, the flow restriction will differ.
The reason for this has to do with how the flow stream of the fluid reacts to a restriction. Just downstream of the geometry itself, is a point where the fluid can actually be more restricted than at the geometry itself. For example, an orifice plate has a hole in it. The flow through that hole has to squeeze through this hole, but it actually must continue to contract or squeeze down past that hole. The point of maximum restriction (minimum flow area) is actually downstream of the actual hole. This point is known as the "vena contracta". So for various restrictions, the geometry itself causes restrictions (a vena contracta) at other locations, downstream of the actual geometry. How the fluid reacts to those geometry is dependant on how the fluid 'sees' the restriction. The example of a well rounded entrance versus a well rounded exit is another good example.
All of these restrictions are well defined by the Crane paper.
