In entangled polymeric liquids, a subchain between two entanglements is an open system, which can exchange Kuhn segments with its neighboring subchains along the given polymer. A subchain is also a small, nonmacroscopic system, in a thermodynamic sense, because the number of segments belonging to it is small. Fluctuations are not negligible, then, and must be properly accounted for. Here, it is demonstrated that the adoption of nonstandard (small system) statistical thermodynamics for the description of the subchain behavior leads to significant corrections to classical Doi-Edwards formulas. The strain measure tensor for step deformations is recalculated in the nonstandard approach and shown to be in excellent agreement with available step shear data.